Compound Interest Calculator

The fields start at 10,000 with 100 added each month at 6% for 10 years, so you can see a full result before typing anything.

1. Enter the Initial deposit, the lump sum you start with. Use 0 to start from nothing.
2. Enter the Monthly contribution you plan to add.
3. Type the Annual interest rate (%).
4. Set Years to a whole number from 1 to 100.
5. Choose Compounding: Monthly, Quarterly, Yearly or Daily.
6. Change the Currency symbol if you like. It changes the sign only.

The headline figure is the balance at the end of the period. The rows beneath list Initial deposit, Total contributions, Total deposited, Interest earned and Growth, which shows the balance as a multiple of what you paid in. A table gives Deposited, Interest and Balance at the end of each year.

Two reading tips help. The Deposited and Interest columns in that table are running totals since day one, not the amounts for a single year, and the year label has a calendar year in brackets, counted from the current year. Also enter the rate as a percentage: 6 means six per cent, whereas 0.06 would be read as six hundredths of one per cent and show almost no growth.

For a single lump sum the formula is A = P × (1 + r/n)^(n × t), where P is the deposit, r the annual rate as a decimal, n the compounding periods per year and t the years. A deposit of 1,000 at 5% compounded monthly for 10 years becomes 1,000 × (1 + 0.05/12)^120, which is 1,647.01.

Monthly contributions have no single tidy formula when the compounding period differs from a month, so the tool steps through every period. With the starting values (10,000 initial, 100 a month, 6%, 10 years, monthly) the balance reaches 34,581.90. You paid in 22,000 (10,000 plus 12,000 of contributions), so interest earned is 12,581.90, and the first year ends at 11,850.33 with 650.33 of interest.

Timing matters. In each period the existing balance earns interest first, then contributions are added. With monthly compounding that is one payment at each month end. With quarterly or yearly compounding, three or twelve contributions are added together at the end of the period, so they earn nothing until the next one. This is a simplification, and it slightly understates growth for yearly and quarterly settings compared with deposits that start earning mid-period. If your real deposits are monthly, choose Monthly or Daily to stay closest to how the money behaves.

The effect of the setting itself is modest. Using the same inputs:

Time and the rate do most of the work. A stated 5% compounds to an effective 5.13% a year when daily, 5.12% monthly, 5.09% quarterly and exactly 5% yearly, so the gap between schedules is small compared with the gap between 5% and 6%. Regular deposits matter too: in the main example, deposits of 22,000 grew to roughly 1.57 times that figure.

For a rough doubling time, divide 72 by the rate: at 6% that is about 12 years, and the exact answer is 11.9. To find the monthly sum needed for a fixed target, use the savings goal calculator, which works from the goal backwards. To see what interest would be without compounding, try the simple interest calculator, and for the return on a finished investment the ROI calculator is the better fit. The percentage calculator helps with the percentage change between two balances.

The model assumes one constant rate, reinvested interest, no withdrawals, no fees, no tax and no inflation adjustment, so the balance is in nominal money. Real accounts and markets vary. Treat the output as an illustration for general information and not as financial advice or a forecast.