Compound Interest Calculator

Find out how much your savings or investments will grow when interest earns interest — with optional monthly contributions.

Quick answer

A compound interest calculator shows how savings grow when interest is added to the balance and then earns interest itself. $10,000 at 5% compounded annually becomes $16,288.95 after 10 years. Regular monthly contributions and more time have a much bigger effect on the result than compounding frequency.

$
$
%
yrs
Future value
$82,705
Total deposits
$46,000
Interest earned
$36,705
Starting with $10,000 and adding $200 each month at 6% a year, you would have $82,705 after 15 years. You put in $46,000; the other $36,705 is interest earning interest. Give it more time and growth eventually outpaces your deposits.

Key takeaways

  • Time is the biggest driver of compound growth.
  • Monthly contributions compound too.
  • The Rule of 72 estimates doubling time: 72 ÷ rate.
Compound interest — Interest calculated on your original deposit plus all interest already earned.

How the compound interest calculator works

Each period, interest is added to your balance, and the next period's interest is calculated on that larger balance. Monthly contributions are added at the end of each month and grow at the equivalent monthly rate.

Formula

A = P(1 + r/k)^(k·t) + PMT × ((1 + i)^(12t) − 1) ÷ i k = compounding periods per year, i = (1 + r/k)^(k/12) − 1 (equivalent monthly rate)

Standard formula. Further reading: Investor.gov (U.S. SEC) – Compound interest calculator · Investor.gov – Compound interest (glossary)

Worked example

$10,000 at 5% compounded annually for 10 years: 10,000 × 1.05^10 = $16,288.95. You earned $6,288.95 without adding anything.

Change any number above and the plain-English explanation updates instantly with your own figures.

Frequently asked questions

What is compound interest?

Interest calculated on both your original money and the interest already earned, so growth accelerates over time.

Does compounding frequency matter much?

A little. Monthly or daily compounding gives a slightly higher result than annual at the same nominal rate, but the rate and time matter far more.

What is the Rule of 72?

Divide 72 by the annual rate to estimate the years to double your money. At 6%, money doubles in about 12 years.

Is APY different from interest rate?

APY includes the effect of compounding. A 5% rate compounded monthly equals about 5.12% APY.

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Results are estimates for educational purposes and are not financial advice.

Written and reviewed by Muhammad Asif, founder of CalcPane · Formulas verified by automated tests · Updated