Compound Interest Calculator
Work out how savings or investments grow with compound interest and regular monthly contributions.
- Final balance
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- Total invested
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- Total interest earned
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Year-by-year growth
| Year | Invested | Interest | Balance |
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How it works
Enter a starting balance, an optional monthly contribution, the annual interest rate and
how many years you plan to save — the compound interest calculator updates the final
balance, the total you put in and the total interest earned as you type. Interest is
applied once per compounding period using the standard formula
A = P(1 + r/n)^(nt), and contributions are added at the end of each period
(a monthly amount is spread evenly across the periods of each year), which is how banks
and brokers usually credit deposits. The year-by-year table shows exactly when compounding
starts to outpace your own deposits, and you can export it as CSV for a spreadsheet.
Every calculation runs in your browser with plain JavaScript — no account, no ads and no numbers sent to a server, so it is safe to model real savings, pensions or loans. Results are estimates: they ignore tax, inflation and fees.
Frequently asked questions
How do you calculate compound interest with monthly contributions?
Start from the compound interest formula A = P(1 + r/n)^(nt), where P is your starting balance, r the annual rate as a decimal, n the number of compounding periods per year and t the number of years. Regular deposits are added at the end of each period and then compound with everything else, so this calculator simulates every period one by one instead of using a single formula — that is why the year-by-year table matches the final balance exactly.
Does compounding daily instead of annually make a big difference?
Less than most people expect. On $10,000 at 7% for 20 years, annual compounding gives about $38,697 while daily compounding gives about $40,551 — roughly 5% more. The rate, how much you contribute each month and how long you stay invested matter far more than the compounding frequency, so use the frequency selector to compare rather than to optimise.
How long does it take to double your money with compound interest?
The rule of 72 gives a quick estimate: divide 72 by the annual interest rate. At 6% your money doubles in about 12 years, at 9% in about 8 years. Set the monthly contribution to zero in this calculator and adjust the years until the final balance is twice the starting balance to see the exact answer for your rate and compounding frequency.