Compound interest calculator

See how savings and investments grow

This compound interest calculator shows what a sum becomes at a fixed rate, with or without regular deposits. It works as a savings calculator and an investment calculator: you get the final balance, your total contributions, the interest earned, a chart and a year-by-year table. Everything runs in your browser.

How to use it

1. Enter the "Initial deposit" and, if you save regularly, the "Regular contribution".
2. Pick the "Contribution frequency" (Monthly or Yearly) and the "Contribution timing": "End of period" or "Start of period".
3. Type the "Annual interest rate (%)", the "Length (years)" in whole years and the "Compounding": Annually, Monthly or Daily.
4. Choose the "Currency": US dollars by default, pounds sterling or euros.
5. Under "Options (optional)", "Annual inflation (%)" shows the balance in today's money, and "Savings goal" works out the monthly contribution you need.
6. "Copy link" creates an address that reopens the simulation with your figures. "Export CSV" downloads the table for a spreadsheet.

Simple interest vs compound interest

With simple interest, only the starting $10,000 earns money. At 5% for 10 years that is $500 a year, so $15,000 at the end. With compound interest, each year's interest earns interest too: $16,288.95 with annual compounding. The $1,288.95 difference is interest on interest, nothing else.

The formula in plain words

The final balance is P × (1 + r/n)^(n × t), where P is the initial deposit, r the annual rate, n the number of compounding periods per year and t the number of years. Regular contributions follow the same logic, each one from the date it is paid in.

Time matters more than the amount

Take two savers who put away $200 a month at 5%, compounded monthly, paid at the end of each month. One starts at 25 and stops at 65: $96,000 paid in, $305,204.03 at the end. The other starts at 35: $72,000 paid in, $166,451.73. Ten years of delay cost $24,000 in deposits but almost $139,000 in final balance. Doubling the effort does not catch up: $400 a month for 20 years gives $164,413.47.

Regular contributions

Save $100 a month for 20 years at 5%, compounded monthly: you pay in $24,000 and end up with $41,103.37 if each deposit lands at the end of the month. At the start of the month, every deposit earns one more month of interest: $41,274.63. A transfer scheduled on payday is close to the second case.

Compounding frequency

The more often interest is added, the faster it grows. For $10,000 at 5% over 10 years: $16,288.95 compounded annually, $16,470.09 monthly and $16,486.65 daily. The gap is real but small: the rate and the length matter far more. Many savings accounts compound daily and pay interest monthly, so "Daily" is the closest setting for them.

Inflation and real return

A dollar in 20 years will buy less than a dollar today. With 2% "Annual inflation (%)", the $41,103.37 from the example above is worth $27,661.39 in today's money. As a quick estimate, the real return is roughly the rate minus inflation: a 3% return with 2% inflation grows your buying power by only about 1% a year.

The rule of 72

To estimate how long money takes to double, divide 72 by the rate. At 6%, 72 ÷ 6 = 12 years. The exact figure with annual compounding is 11.9 years. The calculator shows both under the results, using the compounding you chose: with monthly compounding at 6%, the exact doubling time is 11.6 years.

Reaching a goal

To build $100,000 in 20 years at 4%, starting from zero, compounded monthly with end-of-month deposits, you need $272.65 a month. If your initial deposit already gets you there, the calculator says so.

Limits of the simulation

The calculator assumes the same rate every year. Real returns are never guaranteed: they move up and down and can be negative. Fees reduce your return every year, and how interest and gains are taxed depends on the country and the type of account. Treat the result as a ballpark figure.

Frequently asked questions

What is the difference between "Total contributions" and "Interest earned"?

"Total contributions" adds up the initial deposit and every regular contribution. "Interest earned" is the "Final balance" minus that total.

How are monthly deposits handled with annual compounding?

The calculator uses the monthly rate equivalent to the annual rate, so each deposit earns interest in proportion to the time it stays invested. Money invested for a full year earns exactly the annual rate.

Can I enter a negative rate?

Yes, from -50% to 100%. A negative rate shows how a balance shrinks, and no doubling time is shown.

Are my figures stored?

They stay in this device's browser for your next visit. A copied link contains your figures, nothing else.