What Is Compound Interest?
The Compound Interest Calculator shows how an investment grows over time when interest is earned on both the original amount and the interest already accumulated. Enter an initial amount, an optional monthly contribution, the interest rate (monthly or yearly) and the period, and the tool instantly projects the final balance, the total you invested and the interest earned, plus a year-by-year breakdown. It is perfect for planning savings, retirement or any long-term goal, and it runs entirely in your browser so your figures stay private.
Compound interest is interest earned not only on your original deposit but also on the interest that has already accumulated. Each period, the gains are added to your balance and start earning their own returns, so the money grows faster and faster over time. Albert Einstein is often quoted as calling it the most powerful force in finance — and over decades the compounding effect can dwarf the amount you actually put in.
Where the growth actually comes from
A worked example, so the shape of the curve is concrete: a starting balance of 1,000 plus 100 added every month, earning 0.8% per month. What changes with time is not just the total but how much of it you did not deposit.
| After | You contributed | Balance | Of which interest | Interest share |
|---|---|---|---|---|
| 5 years | 7,000 | 9,275 | 2,275 | 25% |
| 10 years | 13,000 | 22,623 | 9,623 | 43% |
| 20 years | 25,000 | 78,882 | 53,882 | 68% |
| 30 years | 37,000 | 225,253 | 188,253 | 84% |
The contribution column grows in a straight line; the balance does not. Past roughly the twenty-year mark most of the money is interest, which is why starting earlier moves the result far more than contributing more.
Where compound-interest projections go wrong
Mixing a monthly rate with an annual period
Entering 12% as the rate and 10 as the period, when the calculator expects a monthly rate, models 12% a month for ten months rather than 12% a year for ten years. The output looks plausible and is wildly wrong. Always confirm which unit the rate field means before reading the total.
Treating a nominal rate as an effective one
12% a year compounded monthly is not 12% a year. Each month earns 1%, and compounding those twelve months gives about 12.68% — the effective annual rate. The more often interest is credited, the further the effective rate drifts above the nominal one quoted in the advert.
Reading the result in today's money
A projection of 225,000 in thirty years is a nominal figure. What it buys depends on inflation over those three decades. To think in today's purchasing power, either discount the result by expected inflation or run the projection with a real rate — the nominal rate minus inflation — instead of the headline one.
Assuming the return arrives evenly
The formula applies the same rate every period. Real investments do not: the same average return delivered in a different order produces a different ending balance once withdrawals are involved. Treat a projection as the shape of the curve, not as a forecast of any particular year.
The Formula Behind Compound Growth
For a lump sum, the future value is A = P × (1 + r/n)^(n·t), where P is the principal, r the annual rate, n the number of times interest compounds per year, and t the number of years. When you add regular contributions, each one compounds for the time remaining until the end, and the calculator sums every deposit plus its growth to give the final balance.
The key insight is the exponent: because growth is exponential, time matters more than the amount. Investing a smaller sum for longer often beats a larger sum invested late, which is why starting early is so valuable. More frequent compounding (monthly rather than yearly) also nudges the result higher.
Financial Calculations Stay With You
Compound interest is calculated using financial formulas implemented in JavaScript running in your browser. Your principal, rate, and time period are processed entirely locally — your financial data never reaches our servers.