Compound Interest Calculator
See how your savings grow with compound interest and regular contributions.
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How to use the Compound Interest Calculator
Enter your starting balance, monthly contribution, expected annual return and time horizon. The chart shows your balance growing year by year.
The compound interest formula
A = P(1 + r/n)nt — principal P, annual rate r, compounded n times per year for t years. With monthly contributions, each deposit compounds too, which is why the growth curve bends upward so sharply over decades.
Why time beats timing
At 7% average return, money roughly doubles every 10 years (the rule of 72: 72 ÷ rate = years to double). $10,000 invested at 25 becomes ~$150,000 by 65 without adding a cent; starting at 35 gives ~$76,000. Start early, contribute monthly, and let compounding do the heavy lifting. See also our investment calculator and retirement calculator.