Compound Interest Calculator
Watch your money snowball — a free calculator with an animated growth chart, flexible contributions, inflation adjustment, and side-by-side scenario comparison, all in your browser. No sign-up.
How your balance grows
Contributions vs. interest earned, year by year. Hover the chart for exact numbers.
Compare scenarios
Tweak the numbers above, then save up to 3 scenarios to compare side by side.
No scenarios saved yet — adjust the calculator above and click "Add to comparison".
Investing in dividend-paying stocks or funds and planning to reinvest the payouts? Try the dedicated dividend reinvestment calculator instead — it models dividend growth and share price appreciation separately, and shows exactly how much reinvesting adds compared to taking dividends as cash.
How compound interest works
Compound interest is interest earned on both your original balance and on the interest that balance has already accumulated. Each time interest is added, next period's interest is calculated on a slightly larger amount — which is why growth accelerates the longer money is left to compound, like a snowball picking up more snow (and more speed) as it rolls downhill.
This calculator simulates your balance month by month: each month it applies your annual rate (converted to match your chosen compounding frequency), then adds your monthly contribution. That's the same approach used by most retirement and savings calculators, and it's more accurate than a simple end-of-year formula when contributions happen throughout the year.
Why compounding frequency matters less than you'd think
Switching from annual to daily compounding sounds significant, but at typical savings and investment rates the difference over many years is usually small — a percent or two of the total, not a multiple. What moves the final number far more is the interest rate itself, how much you contribute regularly, and how long you leave it invested. Use the frequency selector to see this for yourself.
A worked example
$10,000 invested at a 7% annual rate, compounded monthly with no further contributions, grows to about $20,097 after 10 years — a little more than double, which lines up with the Rule of 72 estimate below. Add a $200 monthly contribution to that same scenario and the 10-year total climbs to about $54,714: $34,000 of that is money you put in yourself, and roughly $20,700 is interest earned on top of it. Plug your own numbers into the calculator above to see the equivalent breakdown for your situation.
What interest rate should I use?
For a savings account, use your account's actual APY. For long-term investing, many people use a long-run historical average (often cited around 7% after inflation for a diversified stock portfolio, or 4-5% for a more conservative mix) — but future returns are never guaranteed.
What's the "Rule of 72"?
A quick mental shortcut for estimating how long money takes to double: divide 72 by your annual interest rate. At 7%, that's 72 ÷ 7 ≈ 10.3 years — close to, but not identical to, the exact answer, since it ignores compounding frequency and any ongoing contributions. Use this calculator when you want the precise figure instead of the shortcut.
Why does inflation adjustment lower the result?
The "in today's dollars" figure discounts your future balance by assumed inflation, showing its purchasing power in today's terms rather than its raw future dollar amount — both numbers are correct, they just answer different questions.
Can I compare more than 3 scenarios?
The comparison chart caps at 3 to stay readable — remove a saved scenario (click its × ) to free up a slot for a new one.