Compound interest calculator
Enter a starting amount, a monthly deposit, a yearly rate and a number of years to see the balance at the end and how much of it is interest. Choose how often the interest is added.
This answer is for the example values in the boxes above. $10,000 to start, $200 a month, 7% a year, interest added every month, for 10 years. Change the boxes to see your own answer.
Deposits are made at the end of each month. The interest is not taxed here, and the balance is not adjusted for inflation.
How it works
Compound interest is interest on interest. The interest earned in one period is added to the balance, and from then on it earns interest too.
Each month the calculator multiplies the balance by one plus the monthly rate and then adds your deposit. You may pick a way of adding interest that is less often than monthly, such as yearly. The monthly rate is then the one that gives the same growth over a year, so the result does not depend on where the months fall. The time you enter is rounded to whole months.
The SEC’s Investor.gov site has a calculator with the same five choices for how often interest is added: yearly, twice a year, quarterly, monthly and daily.
A worked example
You put $10,000 in at 7 percent a year with interest added monthly. After 10 years it is $20,096.61, so the interest has doubled the money.
Now add $200 at the end of every month. After 10 years the balance is $54,713.58. You put in $34,000 and the interest added $20,713.58.
Time does more than the rate
The same $200 a month at 7 percent, with nothing to start, grows to $243,994.20 in 30 years. In 40 years it grows to $524,962.68. The extra ten years add $24,000 to what you put in and about $281,000 to the balance.
What it leaves out
Taxes, fees and inflation are not in the balance, so what it buys later is less than the number suggests. The rate you enter is yours to choose, and no result here is a promise of what an investment will earn.
Sources
- Investor.gov, the SEC’s Compound Interest Calculator, read on 2026-11-01.
Questions
How do I calculate compound interest?
Multiply the balance by 1 plus the rate for the period, once for each period. At 7% a year with interest added monthly, each month multiplies the balance by 1.005833. After 10 years $10,000 becomes $20,096.61.
What is the difference between daily and monthly compounding?
Interest added more often starts earning interest sooner, so the balance ends a little higher. $1,000 at 5% for 10 years becomes $1,628.89 with yearly interest, $1,647.01 with monthly and $1,648.66 with daily.
When do the deposits start earning interest?
Each deposit is made at the end of the month, so it starts earning in the next month. A deposit at the start of the month would earn one more month of interest.
How long does it take to double my money?
The Rule of 72 says to divide 72 by the yearly rate. At 7% that is about 10.3 years. The exact figure is 10.24 years, so the rule is close.
What rate should I enter?
A savings account quotes its rate, and it can change. The return on a fund varies from year to year, so a rate you enter for one is a guess and not a promise. Try a few rates to see the range.
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