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Compound Interest Calculator
Enter a starting amount, how much you add each month and the rate you expect. The calculator shows what you end up with and how much of it is growth.
Example
Start with $5,000, add $200 a month and earn 7% a year, compounded monthly. After 20 years you have $124,379. You paid in $53,000, and the other $71,379 is interest.
How the compound interest calculator works
Compound interest means you earn interest on your interest. Each period, the interest is added to the balance, and the next period’s interest is calculated on the larger amount.
For a single deposit the formula is A = P × (1 + r/n)n×t, where P is the starting amount, r the annual rate as a decimal, n the number of times interest compounds per year and t the number of years.
This calculator also adds your contribution at the end of every month, then applies the same growth to the new balance. That is why the gap between the two lines on the chart widens over time: later years have more money working.
Common questions
Does compounding frequency make a big difference?
Less than most people expect. At 7% for 20 years, monthly compounding beats annual compounding by only a few percent. The rate and the amount of time matter far more.
What rate should I use?
For a savings account, use the rate the bank quotes. For investments, nobody knows the future return, so try a low, a middle and a high figure to see a range of outcomes.
Does this account for inflation or tax?
No. The result is in future money, before tax. To estimate today’s buying power, subtract your expected inflation from the rate you enter.