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Compound Interest Calculator
How much will your money actually become? Enter a starting amount, a monthly contribution, and a realistic return — the chart separates what you deposit from what growth deposits for you.
At this pace, money doubles about every 9.0 years (the Rule of 72).
Balance after 25 years
$358,710
at 8% a year, compounded monthly
Monthly compounding with deposits added at each month-end. Markets don't move in straight lines — treat this as a planning model, not a promise.
New to all of this? Read the plain-English explainer first.
How the math works
Each month, your balance earns one-twelfth of the annual rate, and then your contribution lands on top. Next month's interest is calculated on the new, larger total — interest on interest. Formally, the future value combines a growing lump sum and a growing stream of deposits: FV = P(1 + r/12)^(12t) + C × [((1 + r/12)^(12t) − 1) ÷ (r/12)].
The outputs that matter most: at 8% with $300 a month, ten years produces about $54,900 on $36,000 deposited — but thirty years produces about $447,000 on $108,000 deposited. Notice that the money doesn't triple over three times the time; it grows more than eight-fold. That nonlinearity is the entire point of starting early.
For a full walkthrough of the concept — including why starting at 25 beats starting at 35 even when you invest less — read our compound interest explainer, and if you're deciding where the monthly contribution should live, our index funds guide is the natural next step.
Frequently asked questions
What return should I use in this calculator?
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Use 6–8% for long-term, diversified stock index investing (the S&P 500 has averaged roughly 10% nominal, about 7% after inflation, over the past century). Use the actual APY of your high-yield savings account for cash planning. Conservative inputs make for honest plans.
Does compound interest apply to savings accounts too?
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Yes — banks compound savings interest daily or monthly at the stated APY. The math is identical; only the rate differs. That is why a 4.5% APY high-yield account visibly out-earns a 0.4% big-bank account on the same deposit.
How often is interest compounded here?
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This calculator compounds monthly, the most common convention for both investment projections and bank savings. More frequent compounding (daily) changes results only marginally at these rates.
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