Calcwise / Money and tax / Compound interest calculator
Compound interest calculator
Enter a starting amount, a monthly deposit, the yearly interest rate and the number of years.
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About this tool: Compound interest calculator
Compound interest means you earn interest on your earlier interest as well as on your deposits. The longer the money stays invested, the faster the balance grows, which is why starting early matters more than the size of each deposit.
Deposits are made at the end of each month. If interest is added more or less often than monthly, the calculator converts the rate to an equivalent monthly rate, so the yearly growth matches your choice. Tax, if you enter it, is taken from the interest at the end.
Formula
Effective yearly rate = (1 + r ÷ n)^n − 1, where r is the annual rate and n the number of times interest is added per year. Each month: balance = balance × (1 + monthly rate) + deposit.
Worked example
10,000 to start, 200 a month, 5% a year, 10 years, interest added monthly: you deposit 10,000 + 200 × 120 = 34,000 and end with 47,526.55, of which 13,526.55 is interest. With a 19% tax on interest the tax is 2,570.04 and you keep 44,956.51.
Sources and data
- Formulas and data last reviewed
- Accuracy
- The result is plain maths: with valid input it is exact, and we only round to cents.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is paid only on the original amount. Compound interest is also paid on interest already earned, so growth speeds up over time.
Does the result include inflation?
No. The balance is in future money. Use the inflation calculator to see what that amount could buy in today's prices.
Is the rate guaranteed?
No. Savings accounts and bonds can quote a fixed rate, but investment returns vary. Treat the result as a scenario, not a promise.