Compound Interest Calculator
See how savings grow when interest earns interest.
Good to know. Rule of 72: divide 72 by the interest rate to see roughly how many years it takes to double.
Not financial advice. Uses only the numbers and rates you enter — no live data.
How it works
In plain words
Each period, interest is added to the balance, so next period’s interest is calculated on a bigger number. Monthly contributions are added at the end of each month and start earning interest from then.
The formula
without contributions: A = P × (1 + r ÷ n)^(n × t) with monthly contribution c: A = P(1+i)^m + c × ((1+i)^m − 1) ÷ i where i = (1 + r ÷ n)^(n ÷ 12) − 1 and m = months
Worked example
10,000 at 5% compounded yearly for 10 years: 10,000 × 1.05¹⁰ = 16,288.95. That is 6,288.95 of interest, against 5,000 with simple interest.
Sources
- Brealey, Myers and Allen, Principles of Corporate Finance, chapter 2 The standard formulas for loan payments, compounding and present value, the same as a spreadsheet’s PMT and FV.
Sources checked September 29, 2026. 7 automated tests check its answers on every change.
About the compound interest calculator
Compound interest means you earn interest on your interest. In the first year, 10,000 at 5% earns 500. In the second year, you earn 5% of 10,500, which is 525. The growth speeds up every year, and over decades the effect is dramatic.
Three things drive the result: the rate, the time and regular contributions. Time is the one people underestimate. Money invested at 25 has twice as long to compound as money invested at 45, and the extra years at the end add the most.
The compounding frequency matters less than most people think. At 5%, compounding daily instead of yearly adds only about 0.13 percentage points a year. That’s why banks quote an annual percentage yield (APY or AER): it already includes the compounding, so you can compare accounts fairly.
This calculator assumes the rate stays the same for the whole period and that contributions are made at the end of each month. Real investments go up and down, savings rates change, and taxes and fees reduce returns. Inflation also means future money buys less. Treat the result as an illustration, not a forecast.
Questions people ask
What is the compound interest formula?
A = P(1 + r/n)^(nt), where P is the principal, r the annual rate as a decimal, n the number of compounding periods per year and t the number of years.
How long does it take to double my money?
Roughly 72 divided by the interest rate, in years (the rule of 72). At 6%, about 12 years.
Does compounding frequency make a big difference?
Less than you might expect. At 5%, yearly compounding gives 5.00% a year, monthly 5.12% and daily 5.13%.
Are the results guaranteed?
No. The calculator assumes a constant rate. Real returns vary, and taxes, fees and inflation are not included.