Compound Interest Calculator

See how contributions compound over time.

Project how an investment grows over time with compound interest and optional regular monthly contributions.

How it works

Interest is added to the balance at each compounding period, so future interest is earned on past interest too. Regular monthly contributions are added and grow alongside the starting amount.

When it helps

Shows how time and consistency affect savings, for example when comparing retirement or long-term investment plans.

How the projection is calculated

The tool splits the time into compounding periods — 1, 4, 12 or 365 per year depending on the frequency you pick. In each period, the balance grows by the annual rate divided by the number of periods, and then that period's share of your monthly contributions is added. Repeating this for every period gives the final balance. Total contributed is your starting amount plus all deposits, and total interest is the difference between the two.

A worked example

10,000 invested at 7% a year, compounded monthly, grows to about 40,387 after 20 years without any further deposits. Add 200 every month and the final balance rises to about 144,573 — of which 58,000 is money you put in and roughly 86,500 is interest. The longer the timeline, the larger the share that comes from interest earning interest.

Things the calculator leaves out

Real returns go up and down from year to year, while the calculator assumes the same rate every period, so treat results as an illustration rather than a forecast. It also doesn't include taxes, account fees or inflation. To see the result in today's money, you can subtract an expected inflation rate from the return — for example using 4% instead of 7%.

Frequently asked questions

What is the rule of 72?

A shortcut: divide 72 by the annual rate to estimate the years it takes to double your money.

Are returns guaranteed?

No. Real investment returns vary and can be negative. This is a projection, not advice.

Does compounding frequency make a big difference?

Less than most people expect. At 7% for 20 years, monthly compounding ends about 4% higher than annual compounding. The rate and the length of time matter far more.

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