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ToolWrenSee how an investment grows over time with compound interest and optional regular monthly contributions.
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Compound Interest is a free, private online tool that lets you project investment growth with compound interest and optional monthly contributions. It runs entirely in your browser, so nothing you enter is uploaded to a server.
Privacy: this tool runs entirely in your browser. Your input is never sent to, received by, or stored on any server — there are no uploads and no tracking of what you enter.
Compound interest earns interest on your interest, so growth accelerates over time. Starting early matters more than the rate.
Adding a regular monthly amount dramatically increases the final value, because each contribution also compounds.
Enter the principal, annual rate, term, compounding frequency and monthly contribution to see the future value, total contributed and interest earned.
Compound interest is interest earned on both your original money and on the interest it has already earned. Unlike simple interest, which only ever pays on the starting amount, compounding makes your balance grow at an accelerating rate — which is why Einstein supposedly called it the most powerful force in finance. This calculator projects that growth over time.
Each period, interest is added to the balance, and the next period's interest is calculated on the new, larger balance. The formula is A = P(1 + r/n)^(nt), where P is the principal, r the annual rate, n the number of times interest compounds per year, and t the number of years. More frequent compounding (monthly vs yearly) yields slightly more.
Adding a fixed amount every month dramatically increases the final value, because each contribution also starts compounding. The tool combines the growth of your initial principal with the future value of an ongoing monthly contribution stream, then shows how much of the final figure is your own money versus interest earned — making the effect of compounding vivid.
Because growth compounds over time, the number of years often matters more than the interest rate or the contribution size. Money invested in your twenties has decades to multiply; the same amount invested later has far less time to compound. The calculator lets you change the term to see how powerfully time amplifies returns.
Real investments rarely deliver a fixed annual return — markets fluctuate, and inflation erodes purchasing power. Treat the output as an illustrative projection of how compounding behaves, not a guaranteed forecast. It is ideal for comparing scenarios (different rates, terms or contributions). Everything is calculated locally in your browser.