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Compound Interest Calculator
See how a starting balance and regular monthly deposits grow with compound interest. Adjust the rate, time and compounding frequency and watch the chart update live.
Monthly contributionsInflation-adjustedPrivate, runs locally
Your plan
% / year
years
%
%
Results
Future value
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Total contributions
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Total interest
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In today's money
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Money doubles in
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Growth over time
ContributionsInterest
Year-by-year breakdown
| Year | Deposits this year | Interest this year | Total contributions | Total interest | Balance |
|---|
How compound interest works
With compound interest you earn interest on your interest. In year one you earn interest on what you deposited; in year two you earn interest on your deposits plus last year's interest, and so on. Given enough time, interest can overtake the money you put in, which you'll see on the chart as the green bars outgrow the blue.
The calculator simulates your balance month by month. Your annual rate is converted into an equivalent monthly rate for the compounding frequency you choose, so daily, monthly and yearly compounding are all handled exactly.
Three levers that matter most
- Time. Starting 10 years earlier often matters more than doubling your deposits.
- Rate of return. Small differences compound into large ones. Fees reduce your effective rate.
- Consistency. Regular monthly contributions, raised with your income, do the heavy lifting.
Results are projections that assume a constant rate. Real investments go up and down, and this isn't financial advice.
FAQ
What is compound interest?
Compound interest is interest earned on both your original money and on the interest it has already earned. Over long periods this snowball effect makes balances grow much faster than simple interest.
What is the compound interest formula?
For a single deposit, A = P(1 + r/n)nt, where P is the principal, r the annual rate, n the number of compounding periods per year and t the number of years. Regular contributions add the future value of an annuity on top.
Does compounding frequency matter much?
A little. Daily compounding earns slightly more than monthly, which earns slightly more than yearly, but the rate, the time invested and how much you contribute matter far more.
What is the rule of 72?
Divide 72 by the annual interest rate to estimate how many years it takes money to double. At 8% a year, money doubles in roughly 72 ÷ 8 = 9 years. The calculator shows the exact doubling time for your rate.
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