Compound Interest Calculator
Enter a starting amount, an optional monthly contribution, an interest rate and a timeframe to project how it grows.
How it's calculated
Interest compounds monthly, and any contribution is added each month. The starting amount grows as P × (1+i)^m, and contributions grow as an ordinary annuity: PMT × ((1+i)^m − 1) ÷ i, where i is the monthly rate and m is the number of months.
Worked example
1,000 starting, 100/month, 6% annual, 10 years: projected value is approximately 18,207 — try the calculator above for the exact figure.
Frequently asked
Does this assume monthly compounding?
Yes — interest compounds monthly and contributions are added monthly, which matches how most savings and investment accounts actually work. Annual compounding would show slightly lower growth for the same rate.
Does this account for taxes or investment fees?
No — it projects growth at the rate you enter. Real returns from an actual investment account would typically be reduced by fees and, depending on the account type, taxes.