What is the compound interest formula?

A = P(1 + r/n)^(nt) where P is the principal, r the annual rate, n the compounding periods per year and t the years; with continuous compounding A = P·e^(rt).

Why does the effective annual rate matter?

It makes different compounding frequencies comparable: APY = (1 + r/n)ⁿ − 1. A nominal 3.5% compounded monthly yields slightly more than the same rate compounded annually.

Equal instalments or equal principal?

Equal instalments keep the payment fixed and cost more interest overall; equal principal repays the same principal each month with falling interest, costing less in total but more up front. The tool shows the difference.

Will my bank quote the same numbers?

The formulas match, but day-count conventions, rounding and early repayment can shift the figures by a few units — this tool is for estimating and comparing options.

Can I see how the money grows year by year?

Yes. Use the plot link under the results: compound and simple interest are drawn against years, regular investing shows the monthly balance, and a loan shows the remaining principal — which makes it obvious how much of an early payment is interest.

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💰Interest Calculator - Compound, Simple, Savings and Loan Payments

Four everyday interest calculations: compound interest with six compounding frequencies (including continuous) and the effective annual rate, simple interest, the future value of a monthly savings plan, and the monthly payment on a loan compared against the equal-principal method.

Free · No sign-up · Runs in your browserLast updated: 2026-10-26

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How to use Interest Calculator

1Choose compound, simple, regular savings or loan payment
2Enter the principal, annual rate and term (or the monthly contribution and number of months)
3Read the final amount, total interest or monthly payment

FAQ

What is the compound interest formula?: A = P(1 + r/n)^(nt) where P is the principal, r the annual rate, n the compounding periods per year and t the years; with continuous compounding A = P·e^(rt).
Why does the effective annual rate matter?: It makes different compounding frequencies comparable: APY = (1 + r/n)ⁿ − 1. A nominal 3.5% compounded monthly yields slightly more than the same rate compounded annually.
Equal instalments or equal principal?: Equal instalments keep the payment fixed and cost more interest overall; equal principal repays the same principal each month with falling interest, costing less in total but more up front. The tool shows the difference.
Will my bank quote the same numbers?: The formulas match, but day-count conventions, rounding and early repayment can shift the figures by a few units — this tool is for estimating and comparing options.
Can I see how the money grows year by year?: Yes. Use the plot link under the results: compound and simple interest are drawn against years, regular investing shows the monthly balance, and a loan shows the remaining principal — which makes it obvious how much of an early payment is interest.