Compound Interest Calculator
See what a starting amount and a monthly deposit become over the years at the rate you expect. Type the rate per year or per month, choose how it compounds, and get the final balance, the interest earned and a year by year table you can print.
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Result
Type an amount, a rate and the years to see the growth.
Estimate with a fixed rate. Real investments change rate, charge fees and taxes, and inflation lowers what the balance buys. Generic maths, no country's tax rules or market rates, valid for any year. Estimate, not financial or legal advice.
Year by year
| Year | Deposited so far | Interest so far | Balance |
|---|
Deposits at the end of each month; balances rounded to the cent.
How compound interest is calculated
Interest on interest
Every month the balance earns interest, and next month that interest earns interest too. Over long periods this is what makes the curve bend upward.
Monthly steps
Balance = previous balance × (1 + monthly rate) + monthly deposit. The deposit goes in at the end of the month, so it starts earning the month after.
Per year or per month
A monthly rate is used as it is. An annual rate is turned into a monthly one in one of two ways: nominal (rate ÷ 12, the APR style of US banks) or effective ((1 + rate)^(1/12) - 1, so a full year gives exactly the rate typed).
What this leaves out
Taxes, fees, changing rates and inflation. Use a rate after fees for a closer estimate. The rate is always yours: this page ships no bank or market rate.
Formulas
- Monthly rate (nominal)
- annual rate ÷ 12
- Monthly rate (effective)
- (1 + annual rate)^(1/12) - 1
- Each month
- balance × (1 + monthly rate) + deposit
- Total deposited
- starting amount + deposit × months
- Interest earned
- final balance - total deposited
Small amounts every month add up. Here is one more.
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- Type the starting amount and, if you will keep adding, the monthly deposit.
- Type the rate and say if it is per year or per month. For an annual rate, pick nominal or effective.
- Type the number of years.
- Read the final balance and the interest earned, check the year by year table, print it or download the card.
Examples
10,000 plus 500 a month at 7% a year for 10 years
Nominal rate compounded monthly (0.5833% a month): final balance 106,639.02, of which 70,000.00 deposited and 36,639.02 interest (34.4%). As an effective 7% a year: 105,197.38.
1,000 at 1% a month for 2 years
No deposits. Balance 1,269.73, interest 269.73. 1% a month is 12.68% a year, not 12%.
200 a month at 0% for 5 years
Balance 12,000.00: only what was deposited. A rate of zero shows how much of the growth is interest.
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Questions
What is compound interest?
Interest calculated on the amount plus the interest already earned. Simple interest is calculated on the starting amount only, so it grows in a straight line; compound interest grows faster each year.
What is the formula?
Without deposits: final = amount × (1 + rate)^periods. With monthly deposits the calculator adds each month: balance × (1 + monthly rate) + deposit.
Is 1% a month the same as 12% a year?
No. Compounded, 1% a month is 12.68% a year: (1.01)^12 - 1. That is why the calculator asks whether the rate is per month or per year.
Nominal or effective annual rate?
A nominal rate (APR) is divided by 12 each month, so the year earns a bit more than the rate. An effective rate already includes the compounding. With 10,000 plus 500 a month at 7% for 10 years: 106,639.02 nominal vs 105,197.38 effective.
How long does it take to double my money?
A quick rule: 72 ÷ the annual rate. At 7% a year, about 10 years. The table shows the exact year for your numbers.
Does it include taxes and inflation?
No. Type a rate after fees, and remember that inflation lowers what the final balance buys.
Is my data saved?
No. Your numbers stay in your browser: nothing is uploaded or saved.