Monthly Compound Interest Calculator
Calculate compound interest with monthly compounding — the most common frequency for savings accounts, CDs and most bond investments. Interest is added to the balance every month, then earns interest itself.
Your Inputs
$
$
Future balance
Contributed
Interest earned
How to Use
1. Enter your initial investment — the lump sum you start with today.
2. Add your monthly contribution.
3. Set your expected annual return and time horizon with the sliders.
4. Choose how often interest compounds.
Results and the growth chart update instantly.
2. Add your monthly contribution.
3. Set your expected annual return and time horizon with the sliders.
4. Choose how often interest compounds.
Results and the growth chart update instantly.
Calculation Method
Future value of a lump sum plus a stream of monthly contributions:
FV = P(1 + r/n)^(nt) + PMT × [((1 + rm)^m − 1) / rm]
P = principal, PMT = monthly contribution, r = annual rate, n = compounds/year, rm = r/12, m = months, t = years. Returns are user-assumed and not guaranteed.
FV = P(1 + r/n)^(nt) + PMT × [((1 + rm)^m − 1) / rm]
P = principal, PMT = monthly contribution, r = annual rate, n = compounds/year, rm = r/12, m = months, t = years. Returns are user-assumed and not guaranteed.
Source: Standard compound interest / annuity future-value formula.
Frequently Asked Questions
Why does monthly compounding produce a higher balance than annual?
Each month's interest starts earning its own interest sooner. Over 30 years at 8% the difference between monthly and annual is roughly 9%.
What earns monthly compound interest?
Most US savings accounts, money-market accounts, and many bond funds credit interest monthly.
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Investment Disclaimer: Calculations are estimates based on assumptions. Past performance does not guarantee future results. Investments involve risk, including potential loss of principal. SmartStockCalcs is not a registered investment advisor. For educational purposes only — consult a licensed financial advisor before making investment decisions.
Last updated: June 6, 2026