Compound Interest Calculator
Enter a starting amount, how much you add each month, the interest rate and how long you will invest. We show what it grows to and how much came from interest.
This calculator gives estimates for information only. It is not financial, tax or legal advice. Real results depend on lender terms, fees and local rules.
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The formula
A = P(1 + r/n)^(nt). Each monthly contribution is compounded from the month it is added.
How compounding works
With compound interest you earn interest on your interest. 10,000 at 7 percent becomes 10,700 after a year. The next year you earn 7 percent on 10,700, not on the original 10,000, and so on. Over long periods this snowballs.
A worked example
10,000 invested at 7 percent for 10 years with 200 added each month grows to about 54,714. You paid in 34,000, so roughly 20,700 is interest.
Time is the biggest lever
Starting early matters more than starting big. 200 a month at 7 percent for 30 years grows to about 244,000, of which only 72,000 is your own money. Waiting ten years to start would cut it to roughly 104,000.
How often does it compound?
Daily, monthly, quarterly and yearly options are included. More frequent compounding adds a little: at 7 percent, annual compounding gives 7 percent a year and monthly gives 7.23 percent. The effect is small compared with the rate and the time.
Real investments do not earn a steady return, and inflation reduces what the money buys. Treat the numbers as an illustration.
Examples to check
Type these into the calculator above to see how it behaves. Every result below was produced by the same calculator code that runs on this page.
| What you enter | What you get |
|---|---|
| Initial investment ($): 10000; Monthly contribution ($): 200; Annual interest rate (%): 7; Time (years): 10; Monthly | Future value: $54,713.58 |
| Initial investment ($): 5000; Annual interest rate (%): 5; Time (years): 20; Monthly | Future value: $13,563.20 |
| Monthly contribution ($): 300; Annual interest rate (%): 6; Time (years): 30; Monthly | Future value: $301,354.51 |
Compound Interest Calculator: frequently asked questions
What is the difference between APR and APY?
APR is the stated yearly rate. APY includes the effect of compounding, so it is a little higher if interest is added more than once a year. We show the APY under the result.
Is this financial advice?
No. It gives an estimate from the numbers you enter. Real offers depend on fees, tax and the lender or provider, so check the terms before you commit.
Is what I enter in this calculator saved or sent anywhere?
No. The calculation runs in your browser, so what you enter is not sent to our servers. We only count, anonymously, whether a tool ran successfully.
Is this calculator free to use?
Yes. It is free, needs no sign-up and has no usage limit.
Last reviewed: October 3, 2026