Compound Interest Calculator
Future value, interest earned & growth
How to Use
- 1Enter your starting amount (principal) and the annual interest rate.
- 2Choose how often interest is compounded. Monthly is most common.
- 3Set the number of years and read the future value instantly.
- 4The tiles show interest earned and the effective annual rate.
Compound Formula
- Future valueFV = P · (1 + r/n)ⁿᵗ
- Interest earnedI = FV − P
- Effective annual rateEAR = (1 + r/n)ⁿ − 1
- Rule of 72years to double ≈ 72 ÷ rate%
Quick Examples
Tap an example to load it above.
Everything about the Compound Interest Calculator
How compounding grows your money and why time matters more than anything.
Compound interest is interest that earns interest. Each period, the interest you've already earned is added to the balance, and the next period's interest is calculated on that bigger amount. This compound interest calculator shows the future value of any amount, the total interest earned, and the effective annual rate, instantly.
It's the engine behind savings accounts, fixed deposits, retirement funds and investment growth, and it also works against you on unpaid credit-card balances. Understanding it is one of the most valuable everyday money skills.
Rule of 72: to estimate how long money takes to double, divide 72 by the interest rate. At 6% it doubles in about 12 years; at 9%, about 8 years.
How compound interest is calculated
The formula is FV = P · (1 + r/n)n·t, where P is the principal, r the annual rate (as a decimal), n how many times per year interest compounds and t the years. $10,000 at 5% compounded monthly for 10 years grows to 10,000 × (1 + 0.05/12)120 ≈ $16,470, which is $6,470 of interest.
What grows your money fastest
- 1Time
Compounding accelerates over the years, so starting 10 years earlier usually beats doubling the deposit later.
- 2Rate
Small rate differences snowball: at 30 years, 7% grows your money roughly 3× more than 4%.
- 3Frequency
More frequent compounding helps a little, since monthly vs yearly at 5% adds about 0.12% effective rate.
- 4Consistency
Leaving the balance untouched lets every earned cent keep earning, while withdrawals reset the snowball.
Frequently Asked Questions
What is compound interest?
Interest calculated on the initial principal plus all previously earned interest. Unlike simple interest (which stays flat), compound interest grows faster and faster because each period's earnings join the balance and earn too.
How do I calculate compound interest by hand?
Use FV = P(1 + r/n)^(n·t). Example: $1,000 at 7% compounded yearly for 30 years = 1,000 × 1.07³⁰ ≈ $7,612. The interest earned is the future value minus the principal.
Does compounding frequency really matter?
It helps, but modestly. $10,000 at 5% for 10 years grows to $16,289 compounded yearly, $16,470 monthly and $16,486 daily. The rate and the time horizon matter far more than the frequency.
What is the Rule of 72?
A quick mental shortcut: divide 72 by the annual rate to estimate the years needed for money to double. 72 ÷ 6 = 12 years at 6%. It's remarkably accurate for rates between 4% and 12%.
What is the difference between simple and compound interest?
Simple interest is paid only on the principal, so it is the same amount every year. Compound interest is paid on principal + accumulated interest, so it grows exponentially. Over 30 years at 7%, $1,000 earns $2,100 simple but about $6,612 compound.
Is this compound interest calculator free and does it work on mobile?
Yes. It's 100% free, needs no sign-up and no download. It works in any modern browser and is fully responsive. Your numbers stay in your browser; nothing is stored or shared.