How it works

Compound interest is interest earned on both your original balance and the interest already added. Over long periods this snowball effect is what drives most investment growth.

The lump-sum part uses the standard future-value formula FV = P × (1 + i)n, where P is your starting amount, i is the rate per compounding period, and n is the number of periods. Your monthly contributions are grown separately as an ordinary annuity and added on top, so the total reflects both your deposits and everything they earn.

Worked example

Start with $1,000, add $200 a month at a 7% annual return compounded monthly for 20 years. Your own deposits total $49,000, but the balance grows to roughly $108,000 — the extra ~$59,000 is compound interest doing the work.

Frequently asked questions

Does more frequent compounding really matter?
At typical rates the difference between monthly, daily and yearly compounding is small but real — more frequent compounding earns slightly more. The rate and time horizon matter far more than the compounding frequency.

Are my contributions added before or after interest?
This calculator treats contributions as monthly deposits that then compound, giving a realistic estimate for regular investing into a fund or savings account.

Is my data saved anywhere?
No. Every calculation runs entirely in your browser. Nothing you type is sent to a server, stored, or tracked.