Compound interest calculator
Enter a starting amount, a rate and a number of years to see what compounding does, and add a monthly contribution for the real savings picture.
Balance after 10 years
$32,062.99
$22,000.00 put in, $10,062.99 earned in interest
How to use it
- Enter what you're starting with and the yearly rate. Savings accounts quote an APY; enter that.
- Enter how many years you'll leave it and any monthly contribution.
- Pick how often interest is credited. Most savings accounts credit monthly; CDs and bonds vary.
- 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 this is calculated
Each month your contribution is added to the balance, and at the end of each compounding period the balance earns interest: balance × rate × (months in the period ÷ 12), rounded to the cent, which joins the balance so it earns interest too. The calculator runs that loop month by month for the whole term; with no contributions and monthly compounding it reduces to the textbook A = P × (1 + r ÷ 12)^(12 × years). Illustrative only; investment returns aren't guaranteed.
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).
Rates, sources and limits
Illustrative only. Rates change, and returns on investments are not guaranteed. Not financial advice.
Last reviewed 13 Sep 2026 by the CalcRabbit team. Estimates only, not financial, tax or medical advice.
Frequently asked questions
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.