Compound Interest Calculator

About this compound interest calculator

This calculator projects the future value of a lump-sum investment plus optional recurring contributions, using closed-form compound-interest and annuity formulas. It supports six compounding frequencies (annually through continuous), monthly or annual contributions, ordinary or due payment timing, and an optional inflation adjustment to surface the real future value. Everything runs locally in your browser.

How it works

The principal is grown via A = P · (1 + r/n)^(n·t) — or P · e^(r·t) for continuous compounding. Recurring contributions are summed via the annuity-FV formula PMT · [(1 + i)^N − 1] / i, where i is the contribution-period equivalent of the nominal rate. The two pieces are summed for the total future value; if you supply an inflation rate, the calculator divides the nominal FV by (1 + π)^t to show today's purchasing power.

Assumptions

The calculator assumes a constant annual rate, no taxes or fees, and that all contributions land at the chosen timing (start vs end of period). Real-world returns vary and the result is best read as a planning estimate, not a guarantee.

FAQ

How is APY different from the rate I enter?

The rate you enter is the nominal annual rate. APY (effective annual yield) is what the rate actually earns once compounding kicks in: (1 + r/n)^n − 1, or e^r − 1 for continuous compounding.

Why does compounding more often only help a little?

The marginal gain from increasing compounding frequency shrinks fast. Going from annually to monthly is a meaningful jump; going from daily to continuous adds only a few cents per thousand dollars over decades.

What is 'contribution timing'?

Ordinary annuities pay at the end of each period; annuity-due pays at the start. A due annuity earns one extra period of interest per payment, so its FV is the ordinary FV multiplied by (1 + i).

Is my data uploaded?

No. All math runs in your browser; nothing is sent to a server.