Compound Interest Calculator

How to use the compound interest calculator

1. Enter your initial principal — the amount you already have — and pick your currency.
2. Enter a regular contribution and choose whether you add it monthly or yearly, at the beginning or the end of each period. Leave it at 0 for a pure lump-sum projection.
3. Set the annual interest rate and the compounding frequency your bank or broker uses (annually, semiannually, quarterly, monthly or daily).
4. Choose how many years the money grows, up to 100.
5. Read the final balance, then scroll to the year-by-year table and chart. Use Download CSV to open the schedule in a spreadsheet.

Everything recalculates as you type, and nothing you enter is sent anywhere.

How it works

Compound interest means each period’s interest is added to the balance, so the next period earns interest on a larger amount. For a single lump sum the balance after t years is:

A = P × (1 + r/n)^(n × t)

Regular deposits are the future value of an annuity. With a deposit C made every period and N periods in total, the deposits are worth C × ((1 + i)^N − 1) ÷ i, where i is the rate per period — multiplied by an extra (1 + i) if deposits arrive at the beginning of the period instead of the end.

Method used here. Because your deposit schedule and the compounding schedule often differ — monthly saving into a quarterly-compounded account, for example — this calculator simulates the balance month by month using the equivalent monthly growth factor (1 + r/n)^(n/12). That factor reproduces the textbook formula exactly at every whole year, while letting each deposit grow for the correct fraction of a compounding period. The effective annual rate (APY) reported alongside the balance is (1 + r/n)^n − 1.

Worked examples

Lump sum, monthly compounding. $10,000 at a nominal 5% compounded monthly for 10 years grows to $16,470.09, of which $6,470.09 is interest. The same $10,000 compounded annually reaches only $16,288.95 — the compounding frequency is worth $181.14 here.

Ten years of yearly compounding. $1,000 at 6% compounded annually for 10 years becomes $1,790.85. Simple interest would have paid just $600, so compounding added $190.85.

Saving every month. Start with $5,000, add $300 at the end of every month, earn 7% compounded monthly for 20 years. You pay in $77,000 in total and finish with $176,471.69 — meaning $99,471.69, more than half of the final balance, is interest.

Yearly deposits at the start of the year. $6,000 contributed every January, 6% compounded annually, 30 years: you pay in $180,000 and end with $502,810.06. Switching those deposits to the end of the year would cost you about $28,500 of growth.

Tips and common mistakes

• Nominal rate versus APY. Advertised savings rates are often quoted as APY, while loan and bond rates are usually nominal. If your rate is already an APY, set compounding to annually so it is not compounded twice.
• Deposits do not earn a full year of interest. Money added in month 11 only grows for one month. That is why total interest looks smaller than a rough “rate × average balance” guess.
• A monthly rate is not the annual rate divided by 12 in terms of yield. 12 monthly periods of 0.4167% compound to 5.116% a year, not 5%.
• Small fee differences compound too. A 1% annual fee is not a 1% smaller result — over 30 years it can consume a quarter of the final balance. Subtract fees from your rate before entering it.
• Inflation quietly reduces the outcome. A balance of $500,000 in 30 years buys roughly what $206,000 buys today at 3% inflation. Enter a real (after-inflation) return if you want the result in today’s money.

All figures here are estimates based on a constant rate of return; markets and savings rates change, and nothing on this page is financial advice.

Glossary

• Principal – the starting amount before any interest or deposits.
• Nominal rate – the yearly rate quoted before compounding is applied.
• APY / effective annual rate – the true yearly yield once compounding is counted.
• Compounding period – how often interest is calculated and added to the balance.
• Annuity due – a series of deposits made at the beginning of each period.

Last reviewed: September 21, 2026