A complete walkthrough of how this tool projects your future assets, how each of its three calculation modes works, and how to read the results.
The Compound Interest Calculator is a free, browser-based tool for projecting the future value of monthly contributions. It offers three calculation modes: projecting a future value from your contributions, working backward to find the monthly contribution required to reach a target amount, and working out how long it takes to reach a target amount at a given contribution pace. It also supports splitting a contribution schedule into multiple phases with different amounts or rates, and comparing up to four plans side by side. All calculations run locally in your browser; no input is ever sent to a server.
The calculator compounds monthly. Given an annual rate, it first derives the equivalent monthly rate so that compounding it twelve times reproduces the annual rate exactly:
where r is the annual rate (as a decimal) and i is the monthly
rate used internally. The future value after n months, given an initial
amount P and a fixed monthly contribution C, is then:
The first term is the initial amount compounding on its own; the second is the future value of the stream of monthly contributions (each contribution is assumed to be made at the end of the month and to compound from that point onward). When a contribution schedule has multiple phases with different amounts or rates, the balance at the end of one phase becomes the starting balance for the next, so compounding continues uninterrupted across the whole schedule.
Enter an initial amount, a monthly contribution (optionally split into multiple phases), and an expected annual rate. The tool applies the formula above and shows the projected principal (the amount you contributed), investment profit, and total, along with a year-by-year chart.
Enter a target amount, a time horizon, and an expected annual rate. The tool solves the
same formula for C instead of FV, giving the constant monthly
contribution needed to reach the target by the end of the period. If the time horizon is
zero and the target is greater than zero, there is no solution, and the result is shown as
unattainable.
Enter an initial amount, a monthly contribution, an expected annual rate, and a target amount. The tool solves for the number of years needed to reach the target. There is no artificial cap on this value — if reaching the target would take hundreds or thousands of years at the given pace, the tool reports that figure as-is rather than truncating it. If the contribution and rate can never reach the target (for example, a zero rate with a zero or negative monthly contribution), the result is reported as unattainable rather than showing a misleading number.
Contributing 30,000 per month for 20 years at an assumed 5% annual rate, starting from an initial amount of 0:
The exact figures depend on rounding and are recalculated in the tool itself — this example illustrates the shape of the result, not a guaranteed outcome.
The figures produced by this tool are projections that assume the entered annual rate stays constant for the entire period and that returns are compounded and reinvested every month. They do not guarantee actual investment performance, and taxes and fees are not taken into account. Actual results can differ significantly from these projections due to market fluctuations or changes in interest rates — this is especially true for the required-contribution and required-period modes, where a lower-than-assumed rate of return means the target may not be reached on the projected schedule. This tool does not recommend or solicit the purchase or sale of any specific financial product. Any investment decisions are your own responsibility.