Compound Interest Calculator

How to use this calculator

Enter your starting amount, any regular monthly deposit, the annual interest rate you expect, and how many years you will save for, then press Calculate growth. The tool returns the future value of the account, how much of that is your own contributions, and how much is interest earned. It assumes monthly compounding, which is common for savings accounts and investment funds. Changing the years shows compounding's most important lesson: the longer the money stays invested, the more dramatically the interest portion grows.

The formula explained

With starting amount P, monthly rate i (annual ÷ 12 ÷ 100), N months and monthly deposit PMT, the future value is:

FV = P(1 + i)ᴺ + PMT · ((1 + i)ᴺ − 1) ÷ i

The first term grows your starting amount by compounding it every month. The second term sums up every deposit you make, each one compounding for the months remaining after it is added. Compounding means interest is calculated on a balance that already includes past interest, so the account grows faster and faster — the curve bends upward rather than rising in a straight line.

A worked example

Start with $10,000, add $200 a month, earn 7% a year for 10 years. You contribute $10,000 plus $24,000 in deposits — $34,000 of your own money. With monthly compounding the balance grows to about $54,714, meaning roughly $20,700 is interest. Stretch it to 20 years and the interest portion grows far larger than the deposits, which is the whole point of starting early.

Frequently asked questions

What is compound interest?

Interest earned on both your original money and the interest already added, so growth accelerates over time.

How is it calculated?

The balance is multiplied by one plus the periodic rate each period, with any deposit added before the next period.

Do monthly deposits help?

Yes — they compound alongside your starting amount and substantially raise the final balance.