How it works
The calculator compounds monthly. Each month the balance grows by the average monthly return r, then the monthly contribution is added at the end of the month: new balance = balance × (1 + r) + contribution. Total paid in = starting capital + contribution × months, and gain = ending balance − total paid in.
Worked example with the default values: 10,000 at 2% per month plus 200 a month for 60 months ends at about 55,621. Of that, 22,000 is paid in and about 33,621 is gain. Read the return field carefully: it is a monthly rate. 2% per month compounds to about 26.8% per year, a very high assumption for any strategy, and no return is guaranteed. 0.5% per month is about 6.2% per year.
Limits: the model assumes the same return every month. Real returns vary and can be negative, and the order of good and bad months matters once money is added or withdrawn. Taxes, fees and inflation are not included. Use the result to see how time, contributions and return interact, not as a forecast. For an annual return with costs and inflation, see the ETF savings plan calculator; to turn a start and end value into an annual rate, see the CAGR calculator.