Compound interest calculator

Enter how much you have today, how much you can add each month and an estimated annual return. The calculator shows how much you could have at the end and how much of it comes from interest.

What is compound interest?

Compound interest means earning interest not only on the money you put in, but also on the interest you've already earned. Over time this snowball effect makes growth speed up: slow in the early years, very fast in the later ones.

The formula

For a starting amount with no extra contributions, the formula is:

Final amount = P × (1 + r/n)^{n × t}

P: starting amount (principal).
r: annual interest rate as a decimal (5% = 0.05).
n: number of times interest is compounded per year (12 if monthly).
t: number of years.

Our calculator compounds monthly and adds your contribution at the end of each month, which is how most regular savings plans work.

Why starting early matters so much

With 100 a month at a 5% annual return, after 10 years you'd have contributed 12,000 and have about 15,500. After 30 years you'd have contributed 36,000 but have more than 83,000: over half of it would be interest. Time is the most powerful ingredient in compound interest, and it's the only one you can't get back.

Important: real investment returns aren't fixed and can be negative in some years. Use this calculator to get an idea, not as a promise of results. Remember to account for inflation and taxes too.

Results are estimates for educational purposes. They are not financial advice and do not guarantee future returns.