Compound Interest Calculator
Free to use, with no sign up. Your entries stay in this browser; calculations run on your device.
Result
A = P(1 + r/n)^(nt) + Σ C(1 + r/n)^[n(t − tₖ)]; APY = 100[(1 + r/n)^n − 1]. P = principal, r = annual rate / 100, n = periods/year, C = each contribution, tₖ = its deposit time in years.
OpenStax Contemporary Mathematics: compound interest explains the principal formula. Contributions sum the same growth formula from each deposit time. Simple interest formula. Modeled APY assumes a constant nominal rate and equal compounding periods over one year. US Truth in Savings Act / Regulation DD, Appendix A: APY = 100[(1 + interest/principal)^(365/days) − 1]; the expression above is its one-year, fixed-rate specialization. Actual account disclosures can use different day counts and account terms.
Worked example: $1,000 at 5% compounded monthly for 2 years gives $1,104.94: $1,000 contributed and $104.94 interest. Modeled APY is 5.1162%.
This is fixed-rate arithmetic. Amounts are USD; fees, withdrawals, changing returns, inflation and taxes are excluded. Money inputs accept up to two decimal places. Results round to cents without rounding intermediate interest. Monthly periods are 1/12 year and yearly periods are 1 year. Only complete contribution periods count: deposits occur at their start or end; a fractional tail earns modeled interest but has no partial deposit. Fractional compounding uses exponential interpolation. Zero years means no deposits or interest. The table includes every full year and any final fractional year.
Arithmetic only; not financial advice.
Related calculators: Investment Calculator, CD Calculator, Simple Interest Calculator.
How to use this tool
Enter principal in USD, annual nominal rate as a percentage, and the term in years. Set compounding periods per year (1 annual, 4 quarterly, 12 monthly or 365 daily). Optionally enter recurring deposits and choose monthly or yearly, start or end. Select Calculate to see the balance and yearly table.
Frequently asked questions
Is this calculator free and private?
Yes. Arithmetic runs in your browser without uploading your entries or saving them to browser storage.
Does the annual rate mean APY?
The input is a nominal annual rate (APR), not APY. Compound mode shows modeled APY from that rate and compounding frequency; simple mode has no compound APY.
How are recurring contributions timed?
Monthly or yearly contributions apply only to complete contribution periods. Start deposits are at the beginning of each complete period and end deposits at its end. A fractional tail earns interest without a partial deposit. Simple interest and the CD preset have no recurring deposits.