Compound Interest Calculator
See how a starting deposit and regular monthly savings grow over time, and how much of the final balance comes from interest.
Growth by year
Year-by-year table
What compound interest is
Compound interest means you earn returns on your past returns, not just on the money you put in. The longer it runs, the more the curve bends upward. In the chart above, the gold part of each bar is money you never deposited.
The formula
For a single deposit, the future value is:
A = P × (1 + r ÷ k)k × t
where P is the deposit, r the yearly rate, k the number of compounding periods per year, and t the years. This calculator also adds your monthly contribution at the end of each month, converting the rate to an equivalent monthly rate for whichever compounding you choose.
The rule of 72
Divide 72 by your yearly return to estimate how many years it takes money to double. At 7%, that is about 10.3 years. At 10%, about 7.2 years. It is a quick mental check, accurate enough for returns between roughly 4% and 12%.
Three things that matter more than the formula
- Time. Starting ten years earlier usually beats contributing more later.
- Fees. A 1% yearly fee comes straight off your return. Lower your expected return by your fund's expense ratio to see the real effect.
- Inflation. Results are in future dollars. To estimate today's buying power, subtract expected inflation (often 2% to 3%) from your return.
See how this applies to index investing in S&P 500 Index Funds, Explained.
Returns are not guaranteed. Investments can lose value.