Compound Interest Calculator
See how your money grows with compound interest and regular monthly contributions.
How to use the Compound Interest Calculator
Compound interest means you earn returns on your returns. Enter a starting balance, a monthly contribution, an expected annual return and a time horizon to project your future balance.
Formula
A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) ÷ (r/n)]
Worked example
$10,000 plus $300 a month at 7% compounded monthly grows to roughly $197,600 after 20 years — more than half of it interest.
Frequently asked questions
What is the Rule of 72?
Divide 72 by the annual return to estimate how many years it takes money to double. At 8%, money doubles in about 9 years.
Does compounding frequency matter?
More frequent compounding earns slightly more, but the rate and time horizon matter far more than monthly vs. daily compounding.
Are these returns guaranteed?
No. Investment returns vary year to year; use a conservative rate for planning.
Results are estimates for educational and planning purposes. Always confirm important financial, health or legal decisions with a qualified professional. Last reviewed 2026-09-27.