Compound Interest Calculator
Find out how much your money could grow when interest earns interest — and how much regular monthly deposits add.
What is compound interest?
With simple interest you earn interest only on the money you put in. With compound interest, the interest you earn is added to your balance, and next period you earn interest on that interest too. Over short periods the difference is small, but over decades it becomes the largest part of your balance. Try changing the years from 10 to 30 above and watch the green part of the bar grow.
The compound interest formula
For a single deposit, the future value is:
A = P × (1 + r/n)n·t
where P is the starting amount, r the annual rate as a decimal, n the number of times interest compounds per year and t the number of years. When you also add money every month, the calculator applies the growth month by month and adds each deposit at the end of the month, which matches how most savings plans and automatic investments work.
Example
Suppose you start with $5,000, add $300 every month and earn 7% a year compounded monthly. After 20 years you would have contributed $77,000, but your balance would be roughly $176,000 — about $99,000 of it from interest. That is the power of starting early and staying consistent.
What rate should I use?
- High-yield savings accounts and CDs pay a fixed or variable rate set by the bank, and are insured up to FDIC limits.
- Stock market index funds have historically averaged higher returns over long periods, but they go up and down, and past returns do not guarantee future results.
- To be conservative, run the calculator with a lower rate as well, and remember that inflation reduces what your future dollars can buy.
Frequently asked questions
Does daily compounding make a big difference?
Not much. Going from monthly to daily compounding adds only a tiny amount. The rate, the time and how much you contribute matter far more.
What is the Rule of 72?
Divide 72 by your annual rate to estimate how many years it takes to double your money. At 8%, money doubles in about 9 years.
Are taxes included?
No. Interest and investment gains may be taxed depending on the account. Tax-advantaged accounts like a 401(k) or IRA can let your money compound with fewer taxes along the way.
Last reviewed November 14, 2026.
