Compound Interest Calculator
See how your money grows when interest earns interest. Enter a starting deposit, a monthly contribution, an expected return, and a time horizon — the balance updates instantly.
| Year | Balance | Contributed | Growth |
|---|---|---|---|
| 5 | $28,495 | $22,000 | $6,495 |
| 10 | $54,714 | $34,000 | $20,714 |
| 15 | $91,882 | $46,000 | $45,882 |
| 20 | $144,573 | $58,000 | $86,573 |
| 25 | $219,269 | $70,000 | $149,269 |
| 30 | $325,159 | $82,000 | $243,159 |
- Total contributions
- $82,000
- Total growth earned
- $243,159
- Final balance
- $325,159
Estimate only. Assumes a constant annual return compounded monthly, with contributions made at the end of each month. Real investment returns vary year to year, and taxes and fees are not included.
How compound interest actually works
With simple interest, you earn a return only on the money you put in. With compound interest, each period's earnings are added to your balance, and the next period's return is calculated on that larger balance. Over short stretches the difference is small. Over decades it is enormous, because the growth itself starts doing most of the work. The future value of a single lump sum compounded monthly is:
FV = P · (1 + r)n
where P is your starting deposit, r is the monthly rate (annual rate ÷ 12), and n is the number of months. When you also contribute a fixed amount every month, each contribution compounds from the month it lands, and the calculator adds the standard annuity term PMT · ((1 + r)n − 1) / r on top. You do not need to memorize either formula — the point is that both time and the rate sit in an exponent, which is why starting early matters more than starting big.
A worked example
Say you start with $10,000, add $200 every month, and earn a 7% annual return compounded monthly. After 10 years you would have contributed $34,000 in total, and the balance would sit at about $54,714 — roughly $20,714 of that is growth. Keep going for the full 30 years and the picture changes dramatically: your total contributions come to $82,000, but the balance reaches about $325,159. Nearly $243,159 of the final balance is growth, almost three times what you actually deposited. For comparison, the $10,000 alone with no monthly contributions would have grown to only about $81,165 over the same 30 years.
How to make compounding work harder for you
- Start as early as possible: years in the market matter more than the size of any single deposit.
- Contribute on a schedule: automatic monthly contributions remove the temptation to time the market.
- Reinvest earnings: compounding only works if dividends and interest stay invested rather than being withdrawn.
- Watch fees: a 1% annual fee compounds against you the same way returns compound for you.
- Use tax-advantaged accounts: a 401(k) or IRA lets growth compound without an annual tax drag.
What this calculator does not include
This is a planning estimate, not a prediction. It assumes one constant annual return every year, while real markets swing up and down — the S&P 500's long-run average hides individual years ranging from steep losses to double-digit gains. It also ignores taxes on interest, dividends, and capital gains, as well as fund fees and inflation, all of which reduce your real-world result. Treat the output as a directional target, not a guarantee.
Frequently asked questions
What is the difference between simple and compound interest?+
Simple interest pays a return only on your original deposit, so growth is a straight line. Compound interest pays a return on your deposit plus all previously earned interest, so growth accelerates over time. Over 30 years at the same rate, a compounded balance can end up several times larger than a simple-interest one.
How often is interest compounded in this calculator?+
This calculator compounds monthly, which matches how most savings accounts, money market accounts, and many investment projections work. Monthly contributions are assumed to be added at the end of each month, so each contribution starts compounding the following month.
What annual return rate should I use?+
It depends on where the money sits. High-yield savings accounts have recently paid around 4 to 5 percent, while the U.S. stock market has historically averaged roughly 7 to 10 percent per year before inflation over long periods. Using a conservative number like 6 or 7 percent for stock investments builds in a margin of safety.
Does compounding really matter for small monthly amounts?+
Yes, because time is the biggest input. Even $100 per month at a 7% return grows to roughly $122,000 over 30 years, and more than half of that is growth rather than deposits. Small consistent contributions started early routinely beat large contributions started late.
Is my financial information stored anywhere?+
No. All of the math runs entirely in your browser. Nothing you type is sent to a server, saved, or shared.
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