How this compound interest calculator works

Compound interest is interest calculated on both your initial investment and the interest it has already earned. Enter an initial investment, an interest rate and how often it compounds, and a length of time to see how much your money could grow.

A = P × (1 + r/n)^(n × t)

Where P is the principal, r is the nominal annual interest rate, n is the number of times interest compounds per year, and t is the number of years. You can also add regular deposits or withdrawals, optionally increasing them each year, to model ongoing contributions or a drawdown.

What the result includes

Future investment value and total interest earnedover the full term.
Effective annual rate (APY): the real yearly return after compounding is taken into account, which is always equal to or higher than the nominal rate you entered.
All-time rate of return (RoR) when there are no regular deposits or withdrawals, or time-weighted return (TWR)when there are — TWR isolates the return driven by the interest rate itself from the effect of your own cash flows.
Time needed to double your investment, based on the effective annual rate.
A year-by-year (or month-by-month) breakdown table of deposits, withdrawals, interest, and running balance.

A note on accuracy

This calculator models deposits, withdrawals, and compounding as regular, evenly spaced periods rather than real calendar dates — the same simplification most online compound interest calculators make. It is an estimate for planning purposes, not financial advice. When a very fine compounding frequency (such as daily) is combined with a coarser, non-aligned deposit or withdrawal period (such as monthly), the exact day a contribution lands can differ slightly by convention, which may produce a tiny (well under 0.01%) difference in the final balance.

Frequently asked questions

What is the compound interest formula?

A = P(1 + r/n)^(nt) — your ending balance A equals your principal P grown by the periodic rate r/n, compounded n times per year for t years.

What is APY and how is it different from the interest rate?

APY (annual percentage yield) is the effective yearly return after compounding, while the nominal rate you enter is the rate before compounding. More frequent compounding (e.g. daily vs. yearly) pushes the APY higher than the nominal rate.

What's the difference between rate of return and time-weighted return?

Rate of return (RoR) is simple when there's no money added or removed along the way. Once you add regular deposits or withdrawals, time-weighted return (TWR) is used instead, since it reflects the return produced by the interest rate alone, not inflated or deflated by the timing of your contributions.

Does compounding frequency matter?

Yes — for the same nominal rate, more frequent compounding (daily or monthly vs. yearly) produces a slightly higher effective annual rate and a larger ending balance, since interest starts earning its own interest sooner.

What's changed

Oct 7, 2026Launched.

References

OpenStax — Compound Interest
the compound interest formula A = P(1 + r/n)^(nt) and the effective annual yield formula (1 + r/n)^n − 1 used for the APY result
OpenStax — Annuities
how regular deposits grow, and why deposits at the beginning of each period end up with one extra period of interest compared with deposits at the end

Year	Deposits	Withdrawals	Interest	Balance
0	–	–	–	$5,000.00
1	$0.00	$0.00	$250.00	$5,250.00
2	$0.00	$0.00	$262.50	$5,512.50
3	$0.00	$0.00	$275.63	$5,788.13
4	$0.00	$0.00	$289.41	$6,077.53
5	$0.00	$0.00	$303.88	$6,381.41

Compound Interest Calculator

Yearly breakdown

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