Compound interest calculator

Enter a starting amount, a rate and a number of years to see what compounding does. Add a monthly contribution to see the real savings picture.

Added at the end of each month.

  1. Starting amount$10,000.00
  2. Contributions$12,000.00
  3. Interest earned at 5%, compounded monthly$10,062.99
  4. Balance after 1 year$11,744.63
  5. Balance after 5 years$19,662.51
Balance after 10 years $32,062.99

Illustrative only. Rates change, and returns on investments are not guaranteed. Not financial advice.

Last updated 13 Sep 2026.

How to use it

  1. Enter what you're starting with and the yearly rate. Savings accounts quote an APY; enter that.
  2. Enter how many years you'll leave it and any monthly contribution.
  3. Pick how often interest is credited. Most savings accounts credit monthly; CDs and bonds vary.
  4. Read the final balance and how much of it is interest rather than your own money.

Worked example

$10,000 at 5%, compounded monthly, with $100 a month for 10 years.

The starting amount alone grows to $16,470.09. The contributions add up to $12,000 and grow to $15,592.90. Final balance $32,062.99, of which $10,062.99 is interest.

How it's calculated

Each month: balance = balance + contribution

At the end of each compounding period: interest = balance × rate × (months in period ÷ 12), balance = balance + interest

The calculator runs that loop month by month for the whole term, rounding to the cent each time. With no contributions and monthly compounding it reduces to the textbook formula A = P × (1 + r ÷ 12)^(12 × years).

FAQ

What's the difference between APR and APY?

APY includes the effect of compounding within the year; APR doesn't. A 5% APR compounded monthly is a 5.12% APY. Banks advertise savings rates as APY, so enter that here with monthly compounding and the result will be very close.

Does contributing at the start of the month change much?

Slightly. Each contribution earns one extra month of interest. Over 10 years at 5% on $100 a month that's about $65 more. This calculator adds contributions at the end of the month, the conservative choice.

How long does money take to double?

Roughly 72 ÷ the rate in years: about 14 years at 5%, 10 years at 7%. That's the rule of 72, and the calculator will show you the exact figure.

Is this before or after tax and inflation?

Before both. Interest in a taxable account is taxed as income each year, and inflation of 2% to 3% reduces what the final balance buys. For a real-terms figure, use your rate minus expected inflation.