Returns & Compounding Calculators

Compound Interest Calculator

Calculate how much your principal and monthly contributions grow with compounding, and how much you need each month to reach a target amount.

Calculated in your browser only · inputs are not saved

Enter your values and press Calculate

Projected amount after 20 years

Total principal contributed

Investment gain

Return

How to use

How to use the compound interest calculator

Choose a mode
Choose whether to see the future value, or to work backwards to how much you need each month to reach a target amount.
Enter amounts and return
Enter your initial investment, monthly contribution, expected annual return and period. For long-term stock returns, 5-8% is a commonly used conservative range.
Check the chart
See how principal and gains diverge year by year, and share the result with a link.

How it works

Principal grows using the period return, which is the annual return r divided by the compounding frequency n, and the contribution is added at the end of each month. When the monthly contribution and the compounding frequency differ, the calculation uses the effective monthly return (1 + r/n)^(n/12) − 1.

Future value = P × (1+i)^N + M × [((1+i)^N − 1) / i] (i = effective monthly return, N = number of months)

Real value is the future amount discounted by the inflation rate, showing how much purchasing power it has in today's money.

FAQ

How different are monthly and annual compounding?

Even at the same 7% a year, monthly compounding gives an effective annual return of about 7.23%. The longer the period, the bigger the gap, but in stock investing the 'expected annual return' is itself an estimate, so taking a conservative view with annual compounding is also an option.

What return rate should I enter?

Compare scenarios: the current rate for deposits, about 4-5% for a bond-stock mix, and about 5-8% for long-term stock index investing. If you want to see actual historical data, use the DCA Calculator.

Are taxes included?

This calculator is pre-tax. You can see tax differences by account type in the After-tax Return Comparison by Account tool.

How is the target-amount calculation done?

The amount your initial investment grows to over the period is subtracted from the target, and the monthly contribution needed to fill the remainder is solved with the annuity formula.

Results are estimates based on your inputs and simplified assumptions, and are for reference only, not a basis for investment decisions.