📈 Compound Interest Calculator
Free compound interest calculator with monthly contributions, compounding frequency, a year-by-year table and growth chart. Runs privately in your browser.
Year-by-year table
| Year | Contributed | Interest | Balance |
|---|
How this calculator works
The calculator grows your balance month by month. Each month, the balance earns the monthly equivalent of your annual rate (adjusted for the compounding frequency you choose), and then your monthly contribution is added. The general formula for a single deposit is A = P × (1 + r/n)n×t, where P is principal, r the annual rate, n the compounding periods per year and t the number of years. Regular contributions are layered on top as a growing annuity.
The gold portion of the chart is the interest earned on earlier interest — the "snowball" that makes time the most powerful variable. Notice how the gold band is thin for the first decade and then dominates: that is why starting early often matters more than contributing a lot later.
Returns are assumed constant for illustration. Real markets fluctuate, and fees and taxes reduce the result. Try lowering the return by 1% to see how much a typical fund fee costs you over the full period.
Frequently asked questions
What return should I assume?
Many planners use a conservative long-run real (after-inflation) return of roughly 4–6% for a diversified stock-heavy portfolio and lower for bond-heavy or cash savings. Use several scenarios rather than one optimistic number.
Does compounding frequency matter much?
Less than most people expect. Moving from annual to monthly compounding raises the effective rate slightly; your contribution amount, the rate itself and time invested matter far more.
Is my data saved?
No. The math runs entirely in your browser. Nothing you type is sent to us or anyone else.
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Results are estimates for educational purposes based on the inputs and assumptions shown; they are not a guarantee or a recommendation. Last reviewed October 30, 2026.