Mega Backdoor Roth at Fidelity: The Complete 2026 Walkthrough

You've decided to execute the Mega Backdoor Roth, your 401(k) is administered by Fidelity, and you want the exact mechanics. This article assumes you've already worked through the conceptual material in the Mega Backdoor Roth deep dive — what it is, why the math works, who benefits most, and how it interacts with the overall annual addition limit. What follows is the operational walkthrough: the screens you'll navigate inside NetBenefits, the contribution rate you'll calculate, the conversion toggle you'll enable, and the pitfalls that quietly cost people thousands in unnecessary tax or missed match dollars.

Before any of that, the strategy requires three things at the plan level. First, your plan must allow after-tax contributions beyond the standard employee deferral limit — this is a distinct money source that lives separately from your Pre-Tax and Roth 401(k) buckets. Second, your plan must allow either in-service distributions of that after-tax sub-account OR in-plan Roth conversions of the after-tax money. Third, Fidelity, in conjunction with how your employer designed the plan, must actually support the conversion mechanism in their interface and operations. The recordkeeper being Fidelity is necessary but not sufficient. Plenty of Fidelity-administered 401(k) plans do not allow Mega Backdoor Roth — the employer simply didn't elect those provisions when they set up the plan document. This guide walks through how to verify eligibility, execute the contribution and conversion, and avoid the operational landmines that catch experienced investors off guard.

Verifying Plan Eligibility Inside NetBenefits

Your first job is to confirm that your specific plan actually supports the strategy. Recordkeeper marketing pages and Reddit threads about other people's Fidelity 401(k)s tell you nothing about your plan. The authoritative answer lives in two places: your plan's contribution setup page in NetBenefits, and the Summary Plan Description (SPD) document that your employer is legally required to make available.

Log into NetBenefits at netbenefits.fidelity.com using the credentials tied to your workplace plan. Once you're inside your account, navigate toward the area that controls your ongoing contributions — depending on how your employer has configured the portal, this is typically labeled "Contribution Amount," "Change Your Contribution," or "Manage My Contributions." The page you want shows the breakdown of where each paycheck dollar can be directed. A plan that supports the Mega Backdoor Roth will show a third line item beneath the familiar Pre-Tax and Roth 401(k) options. That third line is labeled some variation of "After-Tax Contributions," and this is the line that matters. If your plan only displays Pre-Tax and Roth 401(k) as contribution destinations — with no separate after-tax field — then your plan does not support the Mega Backdoor Roth in any form. There is no workaround. The strategy is dead until you change employers or your employer amends the plan document.

Plan administrators are inconsistent about the labels they use for this money source. You may see it called "After-Tax Contributions," "Voluntary After-Tax," "Non-Roth After-Tax," "Employee After-Tax," or "True-Up After-Tax." These all refer to the same thing: a separate contribution bucket that uses already-taxed dollars, sits in its own sub-account, and is eligible to be converted to Roth under the right plan provisions. Critically, this is not the same as Roth 401(k). Roth 401(k) contributions count against your $23,500 employee deferral limit. After-tax contributions do not — they live in the space between your deferral plus employer match and the overall annual addition limit. Confusing the two is the single most common conceptual error newcomers make.

Once you see that After-Tax line, you need to confirm the conversion mechanism. Look elsewhere in NetBenefits — typically under a heading related to "Roth Conversions," "In-Plan Conversions," or sometimes buried inside the same Contributions page — for a feature called "Automatic Roth In-Plan Conversion," "Daily Roth Conversion," or similar. If you see that toggle, you are in the best possible configuration. The system will sweep your after-tax contributions into the Roth sub-account on a near-continuous basis, leaving no time for taxable earnings to accumulate. If you don't see an automatic conversion option, look for language about "In-Service Distribution" or "In-Service Withdrawal" of the after-tax sub-account. That's the older alternative mechanism — you periodically withdraw the after-tax money while still employed and roll it to an external Roth IRA.

The Summary Plan Description is the document that governs all of this. You can find it inside NetBenefits under the "Plan Information" or "Plan Documents" area, usually as a downloadable PDF. The SPD will explicitly describe what contribution types are allowed, what the percentage cap on after-tax contributions is, and what conversion or distribution rights exist. When the NetBenefits interface is ambiguous, the SPD is the tiebreaker. It's also the document you can show to Fidelity's Workplace Investing phone team if their representative gives you incorrect information about your plan, which does happen — call center reps handle many plans and occasionally give generic answers that don't match your specific plan document.

Understanding the 2026 Contribution Limits

The Mega Backdoor Roth math hinges on the gap between three IRS limits, and you need to internalize them before setting a contribution rate. For 2026, the employee deferral limit — the combined cap on your Pre-Tax 401(k) plus Roth 401(k) contributions — is $23,500. If you're age 50 or older, you add a $7,500 catch-up. SECURE 2.0 introduced a higher catch-up specifically for ages 60 through 63 of $11,250, which replaces the standard $7,500 in those years. These deferral limits apply to your own salary-deferred money only and do not include employer match or after-tax contributions.

The number that governs the Mega Backdoor Roth is the overall annual addition limit, which the IRS publishes under Section 415(c). For 2026 this is estimated at $70,000 — treat this as an estimate until the IRS finalizes the inflation adjustment. The overall limit is the cap on the sum of every dollar that enters your 401(k) from every source: your employee deferral, your employer match, any employer profit-sharing contribution, and your after-tax contributions. The available after-tax room is simply the overall limit minus the dollars that have already filled the bucket from other sources.

A worked calculation makes this concrete. Suppose your overall limit is $70,000, you max your employee deferral at $23,500, and your employer match works out to $10,000 for the year. Your remaining after-tax capacity is $70,000 minus $23,500 minus $10,000, which equals $36,500. That $36,500 is what you can theoretically push through the Mega Backdoor Roth in this calendar year. If you have additional employer profit-sharing or non-elective contributions on top of the match, subtract those too. If you're a 50-plus catch-up eligible participant, your catch-up amount does not count against the overall $70,000 limit — it sits on top — which slightly changes the framing but doesn't fundamentally alter the after-tax math for most filers.

| Money Source | 2026 Limit (Estimated) | |---|---| | Employee deferral (Pre-Tax + Roth combined) | $23,500 | | Age 50+ catch-up | +$7,500 | | Age 60-63 special catch-up (SECURE 2.0) | +$11,250 | | Overall annual addition limit (415(c)) | $70,000 | | Available after-tax room | $70,000 minus deferral minus match |

One subtle point about catch-up contributions: a non-Roth catch-up dollar does not increase your available after-tax room because catch-ups don't count toward the 415(c) cap. They're effectively bonus Roth/Pre-Tax space layered on top of everything else. This is generally favorable for older participants, but it means your after-tax calculation doesn't change based on catch-up eligibility.

Setting the After-Tax Contribution Rate

With the dollar target in hand, you translate it into a percentage of pay because Fidelity's contribution interface, like every other 401(k) recordkeeper, works in percentages rather than dollar amounts for ongoing payroll deductions. From the main NetBenefits dashboard, navigate to the contribution setup screen — the same one where you control your Pre-Tax and Roth 401(k) percentages — and locate the After-Tax field.

The percentage calculation is straightforward. Divide your target annual after-tax dollars by your gross annual salary, then multiply by 100. If you want to contribute $30,000 of after-tax money and your salary is $150,000, the math is $30,000 divided by $150,000 times 100, which equals 20%. That's the percentage you enter into the After-Tax field in NetBenefits. Some employees prefer to layer in a small buffer — perhaps targeting 95% of the calculated room — to avoid bumping into the overall limit before year-end and having Fidelity reject the last few contributions. Other plans handle the cap gracefully with automatic correction. Read your SPD to see how yours behaves.

Fidelity-administered plans often impose a per-paycheck or annual cap on the after-tax percentage, separate from the IRS limit. A common cap is 25% of pay, though some plans allow as high as 50% or as low as 10%. The cap is set in the plan document, not by Fidelity globally, so you have to check yours. If your cap is too low to fully consume the after-tax room — say your plan caps at 10% but you'd need 20% to hit the IRS limit — you have a few options. You can simply accept the smaller annual contribution, you can request that HR amend the plan provision (a long shot, but occasionally feasible at companies that have several high earners), or you can spread the unused room into the following year only if your overall financial plan can absorb it. Some employees with bonus-heavy compensation structures get around low caps because bonuses count as pay and the percentage applies to the full bonus amount, which can push a large after-tax contribution through in a single pay period.

The percentage applies to gross pay each pay period, and this creates a quirk that catches even careful participants. A mid-year raise can push your annual after-tax total above your intended target. Suppose you set 20% on a $150,000 salary expecting $30,000 for the year, but you receive a 10% raise in July. From July onward your contributions are running at 20% of $165,000, and your annual total ends up closer to $32,000. If the higher contribution stays within plan caps and IRS limits, fine — but if it pushes you over the overall $70,000, Fidelity will refund the excess after year-end, which is annoying but recoverable. Check whether your plan has true-up or auto-correction provisions described in the SPD. Some plans automatically stop after-tax contributions when the overall limit is reached, others let you ride into excess territory and then refund.

Executing the Conversion with Automatic Roth In-Plan Conversion

If your plan offers Automatic Roth In-Plan Conversion — and this has become the dominant mechanism for newer Fidelity-administered plans — you should enable it the moment you turn on after-tax contributions. The two go together. Without conversion, after-tax money simply sits in a sub-account inside the 401(k), earning returns, and any growth becomes taxable later when you do eventually convert.

To enable automatic conversion, look in NetBenefits for a setting that controls in-plan Roth conversions. The exact path varies by plan, but you're looking for a screen that lets you toggle automatic or daily conversion of after-tax contributions to the Roth 401(k) sub-account. Some plans label this prominently on the main contributions page; others bury it under a Roth conversion or in-plan rollover heading. If you can't find it after a careful search, call Fidelity Workplace Investing at the dedicated number listed in your NetBenefits account — not the general Fidelity Brokerage line — and ask the workplace plan team to confirm whether the feature is available and how to enable it. They can sometimes enable it on your behalf if the self-service interface isn't exposing the option.

The tax implication of automatic conversion is the cleanest possible outcome. Your basis — the dollars you contributed — converts to Roth completely tax-free, because you already paid income tax on those dollars before they entered the 401(k) (that's what "after-tax" means). Any earnings that accumulated between the moment your contribution hit the after-tax sub-account and the moment it converted are taxable as ordinary income in the year of conversion. With automatic same-day or next-day conversion, the dollars are in the after-tax sub-account for a matter of hours, so earnings are effectively zero. The tax bill is effectively zero. This is the entire point of the strategy: you move large amounts of money into Roth territory without paying a meaningful additional tax bill.

Without automatic conversion, the picture deteriorates quickly. If your plan only allows conversions quarterly or annually, your after-tax contributions sit in the sub-account earning market returns between conversion events. Over a quarter, with a strong market, you might accumulate several percent of growth on the balance. When you convert, that growth is taxable. On a $30,000 annual contribution growing at, say, 8% annualized, quarterly conversions could generate several hundred to a few thousand dollars of taxable earnings — not catastrophic but not nothing, and entirely avoidable with automatic conversion. Use the /tools/compound-interest-calculator to see how even small earnings accumulations compound over a working career.

The Alternative: In-Service Distribution to an External Roth IRA

If your plan supports the Mega Backdoor Roth but lacks Automatic Roth In-Plan Conversion, the alternative mechanism is the in-service distribution. You contribute after-tax dollars to the 401(k), let them accumulate (briefly, ideally), then withdraw the after-tax sub-account while still employed and roll the basis to an external Roth IRA. This is the older method, and while it's less common at modern Fidelity-administered plans, it remains the only path forward for some plan designs.

The process starts with a phone call to Fidelity Workplace Investing. Find the dedicated 401(k) phone number in NetBenefits — it's usually displayed prominently near the top of your account dashboard or in the contact section. Tell the representative you want to initiate an "in-service distribution of after-tax contributions" or "in-service rollover of the after-tax sub-account." Be specific that you only want the after-tax sub-account, not the pre-tax or Roth 401(k) balances. The rep will confirm your plan allows this and walk through the paperwork.

You'll need destination account information ready before the call. The basis portion of the distribution should go to an external Roth IRA. If both the 401(k) and the receiving Roth IRA are at Fidelity (Brokerage), the transfer can be processed as a direct internal movement, which is faster and cleaner than a paper check. If the receiving Roth IRA is at a different institution, you can have Fidelity issue a check made payable to that institution for your benefit, then forward it. The earnings portion of the distribution — the taxable growth that accumulated in the after-tax sub-account between contribution and distribution — needs a separate destination. Most participants roll the earnings to a Traditional IRA to defer the tax, though you can also roll the earnings to the Roth IRA and pay the tax now if you prefer to keep everything in one account. The choice depends on your current marginal rate and your projection of future rates.

Once the basis arrives in the external Roth IRA, it's treated as a Roth conversion for purposes of the five-year rule. Each conversion has its own five-year clock for accessing the converted principal penalty-free before age 59½. This generally only matters if you plan to use the Roth IRA for pre-retirement withdrawals — for most participants who treat the Roth as long-term retirement savings, the five-year rule is academic. You can model how that growth compounds over time using the /tools/cagr-calculator.

The Pro-Rata Rule and Why It Doesn't Wreck the 401(k) Version

The pro-rata rule is one of the more confusing pieces of tax law that applies to Roth conversions, and the rule operates differently inside a 401(k) than it does across IRAs. Understanding this distinction is critical because it's exactly what makes the Mega Backdoor Roth work mechanically when the regular Backdoor Roth would otherwise be sabotaged.

Inside a 401(k), the IRS treats each money source as a separate sub-account for pro-rata purposes. Your after-tax sub-account is distinct from your pre-tax sub-account, which is distinct from your Roth 401(k) sub-account. When you convert the after-tax sub-account to Roth — whether via in-plan conversion or in-service distribution to an external Roth IRA — only the after-tax money is included in the conversion. The pre-tax money sits untouched in its own sub-account and is not pulled into the conversion calculation. The earnings inside the after-tax sub-account are taxable upon conversion, but the much larger pre-tax balance is irrelevant to the math. This sub-account separation is what allows the Mega Backdoor Roth to push large dollars to Roth territory without triggering a massive tax bill on commingled pre-tax balances.

Contrast this with the regular Backdoor Roth — the Traditional IRA to Roth IRA strategy used by high earners who exceed the direct Roth IRA contribution income limits. In the IRA world, the pro-rata rule aggregates ALL of your Traditional IRA balances across every account at every institution as of December 31 of the conversion year. If you have a SEP-IRA, SIMPLE IRA, or any rollover Traditional IRA with pre-tax money in it, a Backdoor Roth conversion is partially taxable in proportion to the pre-tax share of your total IRA balances. Many high earners with old rollover IRAs find their Backdoor Roth strategy effectively neutralized by pro-rata, and the workaround usually involves rolling those pre-tax IRA balances INTO a 401(k) to remove them from the IRA aggregation calculation.

The Mega Backdoor Roth, because it operates entirely inside the 401(k) until conversion, sidesteps this landmine completely. Your pre-tax 401(k) balance — even if it's $500,000 — does not get pulled into the conversion of your after-tax sub-account. This is one of the strategy's structural advantages and a major reason it's so powerful for high earners who already have significant pre-tax retirement balances.

A Complete Worked Example

Consider a 35-year-old software engineer earning $180,000 in salary, with a 6% employer match on a Fidelity-administered 401(k) that offers after-tax contributions and Automatic Roth In-Plan Conversion. Walking through her full setup illustrates how the pieces combine.

She maxes her employee deferral at $23,500 in pre-tax 401(k) contributions, planning to manage her taxable income downward into a lower bracket. Her employer matches 6% of salary, which works out to $10,800. Her overall annual addition limit is $70,000, so her available after-tax room is $70,000 minus $23,500 minus $10,800, which equals $35,700.

To target $35,700 in after-tax contributions across the year, she divides $35,700 by her $180,000 salary, gets 0.1983, and rounds to 20%. She enters 20% in the After-Tax field in NetBenefits. The contribution math is slightly imprecise — 20% of $180,000 is $36,000, which is $300 over her actual room. Her plan's SPD states that after-tax contributions automatically stop when the overall limit is reached, so Fidelity will simply halt her contributions late in the year once $35,700 has been deposited. She enables Automatic Roth In-Plan Conversion the same day she sets the contribution percentage. The toggle ensures every after-tax dollar moves to Roth within hours of hitting the sub-account.

The result, year after year: $35,700 of additional Roth contributions, with effectively zero taxable earnings and effectively zero additional tax bill. Compare this to a typical high earner who maxes only the standard $7,000 Roth IRA via the Backdoor Roth — our engineer is moving five times that amount into tax-free territory annually. Over 25 years at a 7% real return, that $35,700 annual contribution stream compounds to roughly $2.4 million in additional Roth assets, sitting on top of whatever her regular pre-tax 401(k) and personal Roth IRA accumulate. By age 60, she has a multi-million-dollar pool of money that will never be taxed again. You can replicate this projection using the /tools/compound-interest-calculator — input $35,700 as the annual contribution, 7% as the rate, and 25 years as the period.

The strategy's leverage compounds over a career. An engineer who starts the Mega Backdoor Roth at 30 and continues to 60 — 30 years of contributions — accumulates substantially more, simply because the early contributions have more time to compound. The earlier you start, the more of the final Roth balance is growth rather than basis.

Pitfalls to Avoid

The first and most common pitfall is forgetting to enable Automatic Roth In-Plan Conversion when it's available. Some participants set up their after-tax contributions, watch the dollars accumulate, and only realize months later that the conversion never happened automatically. The after-tax sub-account is now sitting with several thousand dollars of earnings, all of which will become taxable when finally converted. Always enable automatic conversion as your first action, before the first paycheck deduction processes.

The second pitfall is plan caps on after-tax percentages that prevent you from reaching the overall IRS limit. If your plan caps at 10% of salary, a $200,000 earner can contribute at most $20,000 in after-tax money — well below the typical $30,000-$36,000 available room. There's no clean workaround inside the plan. Some employees front-load by directing their year-end bonus contribution to the after-tax bucket, because a large bonus paid as a percentage of pay generates a large absolute dollar contribution in one event. Others advocate with HR to raise the plan cap. The simplest acceptance is to capture as much as the cap allows and recognize the rest of the room is forfeited for the year.

The third pitfall is front-loading deferrals and losing the employer match for the back half of the year. This pitfall doesn't directly affect the Mega Backdoor Roth, but it interacts with it. If your plan does NOT have a true-up provision, an employer match calculated per-paycheck means you only earn match on paychecks where you actually contribute. If you front-load and hit your $23,500 deferral by July, you contribute zero in August through December, and you receive zero match in those months too. You leave thousands of dollars of free money on the table. Check your SPD for true-up language — true-up plans calculate the match on an annual basis at year-end and make up any missing match dollars. If your plan has no true-up, spread your deferral evenly across all 26 (or 24, or 12) pay periods to maximize match.

The fourth pitfall is highly-compensated employee (HCE) testing failures. Some plans fail the IRS's non-discrimination testing on after-tax contributions and respond by refunding a portion of the after-tax contributions to HCEs after year-end. The HCE threshold for plan year 2024 was $155,000 in compensation, indexed upward — call it roughly $160,000-$165,000 for plan year 2026 once finalized. Large employers with many rank-and-file participants rarely fail testing because the broad participation evens out the contribution rates. Smaller employers, or those where mainly high earners use the after-tax bucket, fail more frequently. There's nothing you can do as an employee to fix testing failure other than to accept the refund and treat it as a return of basis. Use the /tools/roi-calculator when comparing what the Mega Backdoor Roth would have generated against what an alternative use of those dollars (taxable brokerage investing, for example) might return.

The fifth pitfall is leaving your job mid-year with unconverted after-tax money in the sub-account. When you separate from service, you typically have the right to roll the entire 401(k) balance to an IRA or to your new employer's plan. The after-tax sub-account requires careful handling — the basis (your contributions) rolls to a Roth IRA tax-free, and the earnings need to either roll to a Traditional IRA (to defer tax) or to the Roth IRA (and pay tax in the rollover year). A botched rollover where the basis ends up in a Traditional IRA creates a tax mess that requires Form 8606 tracking forever, and a botched rollover where you take a cash distribution rather than a direct rollover triggers immediate taxation and possibly early-withdrawal penalties. Coordinate with Fidelity Workplace Investing on the rollover instructions and confirm the destinations in writing before signing distribution paperwork.

Fidelity vs. Vanguard vs. Schwab: Does the Recordkeeper Matter?

The Mega Backdoor Roth depends on your employer's plan design, not on which recordkeeper administers the plan. Two employers can both use Fidelity as their recordkeeper, and one allows MBDR while the other doesn't, purely because of plan document elections the employer made. That said, the recordkeeper does influence the user experience, the speed of conversion processing, and the availability of certain features.

Fidelity's NetBenefits is generally considered cleaner and more feature-rich for Mega Backdoor Roth execution than Vanguard's workplace plan interface as of the 2024-2026 era. Fidelity has invested heavily in self-service tools for after-tax contributions and in-plan Roth conversions, and the Automatic Roth In-Plan Conversion feature is widely available at Fidelity-administered plans where the employer has elected it. Vanguard has been catching up but historically required more phone calls and manual paperwork to execute conversions, and the Automatic In-Plan Conversion feature appeared at fewer Vanguard plans. Schwab Retirement Plan Services varies more by employer — some Schwab-administered plans have excellent MBDR support, others have notable friction. Empower (formerly MassMutual Retirement and several other consolidated brands) and Principal also support MBDR at many plans and have steadily improved their conversion interfaces.

The practical conclusion is that you should not switch employers solely to chase a different recordkeeper. The plan provisions — what contribution types are allowed, what conversion mechanisms exist, what caps apply — are the variables that matter. A Schwab plan with strong MBDR provisions beats a Fidelity plan that doesn't allow after-tax contributions. Evaluate the plan first; the recordkeeper is a secondary consideration.

What If Your Plan Does Not Support Mega Backdoor Roth

The most common outcome when investors check their plan is disappointment: there's no After-Tax line in the contribution interface, the SPD makes no mention of after-tax contributions, and a phone call to Fidelity Workplace Investing confirms the plan doesn't support the strategy. Several options remain.

First, advocate with HR or your benefits team. Mega Backdoor Roth support has become more common at large employers over the past few years, particularly in tech, finance, and consulting where high-earning employees often request it. A well-framed request — pointing out that adding after-tax contributions and in-plan Roth conversion is a low-cost plan amendment that benefits high earners disproportionately and improves the company's compensation competitiveness — sometimes succeeds. The change cycle is typically annual, so be prepared to wait until the next plan year for any amendment to take effect.

Second, maximize what you do have. Fully fund your Pre-Tax or Roth 401(k) deferral at $23,500. Capture the full employer match. Fund a Roth IRA via the Backdoor Roth if your income exceeds the direct contribution limits. Use /tools/drip-calculator to model how dividend reinvestment in taxable accounts compounds over decades, which becomes more important when tax-advantaged space is limited.

Third, consider whether a job change to an employer with stronger 401(k) provisions makes financial sense over a multi-year horizon. The differential is real — an engineer pushing $35,000 annually through the Mega Backdoor Roth for 20 years has a vastly different retirement picture than an engineer who couldn't. But don't change jobs solely for the 401(k); compensation, growth opportunity, work environment, and other factors typically dominate. Treat MBDR availability as a meaningful tiebreaker between otherwise comparable offers.

Fourth, maximize the Health Savings Account if you have access to a qualifying high-deductible health plan. The HSA is the only triple-tax-advantaged account in the U.S. tax code — deductible going in, tax-free growth, tax-free withdrawals for qualified medical expenses. After age 65, non-medical withdrawals are taxed as ordinary income (like a Traditional IRA), making the HSA effectively a stealth retirement account with superior tax treatment. The 2026 HSA limits are estimated at $4,400 individual and $8,750 family.

For broader strategy context on alternatives and how MBDR fits the overall tax-advantaged hierarchy, return to the Mega Backdoor Roth deep dive.

Frequently Asked Questions

Does Fidelity allow the Mega Backdoor Roth?

Fidelity as a recordkeeper supports the features required for the Mega Backdoor Roth — after-tax contributions and in-plan Roth conversion — but whether your specific Fidelity-administered plan offers them is decided by your employer's plan document. Verify by logging into NetBenefits and looking for an After-Tax contribution line and an in-plan conversion feature, and confirm by reading the Summary Plan Description.

How do I set up after-tax contributions at Fidelity?

Log into NetBenefits, navigate to the contribution setup page (often labeled "Change Your Contribution" or "Contribution Amount"), and locate the field for After-Tax Contributions. Enter a percentage of pay calculated by dividing your target annual after-tax dollars by your annual salary and multiplying by 100. Save the change and confirm the new percentage appears on your account summary.

What is Automatic Roth In-Plan Conversion at Fidelity?

Automatic Roth In-Plan Conversion is a plan feature that automatically converts after-tax contributions to the Roth 401(k) sub-account on a continuous basis — typically same-day or next-day. This minimizes the earnings that accumulate in the after-tax sub-account before conversion, which in turn minimizes the taxable portion of each conversion to effectively zero. Enable this feature in NetBenefits if your plan supports it.

Can I do the Mega Backdoor Roth at Fidelity without an employer 401(k)?

No. The Mega Backdoor Roth requires an employer-sponsored 401(k) plan that allows after-tax contributions and either in-plan Roth conversion or in-service distribution of the after-tax sub-account. You cannot execute the strategy in an individual Roth IRA, a SEP-IRA, or any other personal retirement account. The strategy exists because the 401(k) annual addition limit ($70,000 in 2026) is much higher than the employee deferral limit ($23,500), and the gap can be filled with after-tax money.

How much can I contribute to the Mega Backdoor Roth in 2026?

The maximum is the overall annual addition limit ($70,000 estimated for 2026) minus your employee deferral (Pre-Tax plus Roth 401(k) combined) and minus any employer match or profit-sharing contributions. For a high earner maxing the $23,500 deferral with a $10,000 match, the after-tax room is roughly $36,500. Catch-up contributions for participants age 50 and older do not consume the overall annual addition limit.

What if my Fidelity 401(k) doesn't show an After-Tax option?

If NetBenefits doesn't display an After-Tax contribution line, your plan does not support the Mega Backdoor Roth. Confirm by reading the Summary Plan Description and calling Fidelity Workplace Investing. If the plan truly doesn't allow it, your options are to advocate with HR for a plan amendment, maximize alternatives like Roth 401(k) and HSA, or consider whether a future employer might offer the feature.

Do after-tax contributions get matched by my employer?

Generally no. The employer match is calculated against your pre-tax or Roth 401(k) deferrals up to the match formula limit (typically 3-6% of pay). After-tax contributions sit above the deferral and match and don't trigger additional match dollars. A small number of unusual plan designs match on after-tax contributions, but this is rare — assume no match unless your SPD explicitly states otherwise.

Can I do both Mega Backdoor Roth and regular Backdoor Roth?

Yes. The two strategies operate in completely separate parts of the tax code and don't interfere with each other. The Mega Backdoor Roth fills the 401(k) annual addition gap; the regular Backdoor Roth fills the $7,000 Roth IRA limit for high earners blocked by income phaseouts. A high earner maxing both moves roughly $42,000-$43,000 into Roth annually beyond the standard $23,500 employee deferral. Note that the regular Backdoor Roth still requires care around the IRA pro-rata rule, while the Mega Backdoor Roth does not.

What happens to after-tax contributions if I leave my job?

When you separate from service, the after-tax sub-account is portable along with the rest of your 401(k) balance. The basis (your contributions) rolls tax-free to a Roth IRA. The earnings can roll to a Traditional IRA (to defer the tax) or to the Roth IRA (paying tax in the rollover year). Coordinate the split rollover with Fidelity Workplace Investing carefully — a botched rollover that mixes basis and earnings into the wrong destination creates a tax mess that's hard to unwind.

How is the Mega Backdoor Roth taxed?

The contribution itself uses after-tax dollars, so no deduction is taken when the money enters the 401(k). The conversion of basis from the after-tax sub-account to the Roth sub-account or external Roth IRA is tax-free. Any earnings that accumulated in the after-tax sub-account between contribution and conversion are taxable as ordinary income in the year of conversion. With Automatic Roth In-Plan Conversion, those earnings are effectively zero, making the net tax impact of the strategy effectively zero. Roth funds then grow tax-free and are withdrawn tax-free in retirement.

This article is for educational purposes only and does not constitute investment, tax, or financial advice; verify your plan's specific provisions in the Summary Plan Description, consult with the Fidelity Workplace Investing benefits team, and confirm with a qualified CPA before executing Mega Backdoor Roth conversions.