A stock split is a non-taxable event, but it does change your per-share cost basis. Get the adjustment wrong and you can pay tax on phantom gains.
Forward Split (2-for-1)
Your share count doubles; your per-share basis halves. Total basis is unchanged.
Example: you bought 100 shares at $80 (basis $8,000). After a 2-for-1 split you own 200 shares, with a per-share basis of $40 each. Total basis: still $8,000.
Forward Split (3-for-1, 4-for-1, etc.)
Same logic. Multiply share count by the ratio numerator; divide per-share basis by the same number.
Reverse Split (1-for-10)
Your share count divides by 10; your per-share basis multiplies by 10. Total basis is unchanged.
If the result includes fractional shares your broker can't issue, you receive cash in lieu. That tiny cash-in-lieu payment is a taxable event — typically a small short-term capital gain.
Cash-and-Stock Splits (Rare)
Sometimes a corporate action is a mix: some new shares, some cash. The cash portion reduces your basis. The IRS publishes guidance for big-name examples (Verizon spin-offs, Comcast/Time Warner splits, etc.).
What Total Basis Looks Like
Forward and reverse splits never change your total cost basis. They redistribute it across more or fewer shares. The IRS only cares about total basis when you eventually sell.
A Worked Example
You bought 50 shares at $200 = $10,000 basis. 4-for-1 split → 200 shares at $50 each = still $10,000 basis. You sell 100 shares for $8,000. Profit = $8,000 − (100 × $50) = $3,000.
Plug arbitrary splits into our Stock Cost Basis Calculator — enter the post-split share count at the adjusted per-share basis.
Bottom Line
Adjust per-share basis inversely to the split ratio. Total basis never changes. Cash in lieu is a small taxable event you don't want to forget at tax time.
Frequently Asked Questions
How does a stock split affect my cost basis?
A forward split increases your share count and proportionally decreases your per-share cost basis, while a reverse split decreases your share count and proportionally increases your per-share basis — in both cases, your total cost basis stays exactly the same. For example, 100 shares bought at $80 (an $8,000 total basis) become 200 shares at a $40 per-share basis after a 2-for-1 split, still totaling $8,000. This adjustment matters for correctly calculating gain or loss when you eventually sell.
What happens if a reverse split leaves me with a fractional share?
If a reverse split results in a fractional share your broker can't issue, you typically receive a small cash payment instead ("cash in lieu"), and that payment is generally a taxable event, usually treated as a small short-term capital gain. It's a detail many investors overlook at tax time since the dollar amounts involved are often minor. Keeping records of any cash-in-lieu payments helps avoid discrepancies when filing taxes.
Is a stock split itself a taxable event?
No, forward and reverse stock splits are non-taxable events in themselves — they simply redistribute your existing cost basis across a different number of shares without creating a gain or loss. The IRS only becomes relevant to your basis when you actually sell shares, at which point you use your adjusted post-split basis to calculate the taxable gain or loss. Rarer cash-and-stock hybrid corporate actions are an exception that can create a small taxable component.
Run the numbers yourself
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