Compound interest calculator
See how a starting balance and a monthly contribution grow at a given return over the years, and how much of the final amount is interest on interest rather than money you put in.
- Total contributions
$70,000$70,000 - Interest earned
$100,619$100,619 - Share that is interest
- 58.97%
- Contributions
- Interest earned
Estimates, rounded for display. Rates, taxes and fees change; check the figures that matter against your own statements.
The math behind the result
Interest is paid on the balance, and once paid it becomes part of the balance, so the next payment is on the interest too. Left alone, that turns steady growth into a curve that bends upward, and the longer the money stays, the steeper the bend.
The calculator converts the annual return to the rate for each compounding period, grows the balance every month at the equivalent monthly rate, and adds the contribution at the end of the month. Contributions and interest are tracked separately, so the chart shows how much of the final balance you put in and how much the growth added.
Time does more than rate. Doubling the years does far more than doubling the return, because the late years compound on everything the early ones built. Starting sooner with less usually beats starting later with more.
balance after a month = balance × (1 + r) + contribution r = (1 + annual rate ÷ k)^(k ÷ 12) − 1 · k = times interest is credited a year
Questions, answered.
Connect the savings and brokerage accounts the money sits in and Zypper tracks their balances daily and adds them into your net worth, so the curve you drew here becomes a line you can watch.