Calculate compound interest on your savings
Enter your starting amount, your contributions, the interest rate and the number of years: your final balance, the interest earned, the chart and the year-by-year table appear instantly.
- Regular contributions
- Year-by-year chart
- Doubling time
- Free
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How do you calculate compound interest?
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Enter your savings
A starting amount and, if you like, a regular monthly or yearly contribution, added at the end of each period.
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Set up the investment
Annual interest rate in %, number of years, and interest compounded yearly or monthly.
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Read the results
Final balance, total contributions and interest earned, with a chart and a table showing every year.
Watch your savings grow, year after year
Regular contributions
Add an amount every month or every year to simulate a regular savings plan.
Compounding of your choice
Interest added to the balance once a year or every month, to match the terms of your account or investment.
Chart and table
Stacked bars that separate your contributions from the interest, and a detailed year-by-year table.
Doubling time
The number of years it takes to double your money, using the rule of 72 and the exact calculation.
Simple interest vs compound interest
With simple interest, you only earn interest on your starting amount. With compound interest, the interest is added to your balance and earns interest in turn. The formula is P × (1 + r)^n, where P is the principal, r the rate per period and n the number of periods. Example: $10,000 invested at 5% a year for 10 years earns $5,000 of simple interest, for a total of $15,000, but grows to $16,288.95 with interest compounded annually. The gap widens over time: at the same rate, $10,000 becomes $26,532.98 in 20 years and $43,219.42 in 30 years. That's why time often matters more than the amount you start with.
Calculating savings with regular contributions
Each contribution also earns compound interest, for a shorter time the later it is made. For contributions C made at the end of each period, the formula becomes P × (1 + r)^n + C × ((1 + r)^n − 1) / r. Example: $100 a month for 20 years at 5% a year, compounded monthly, grows to $41,103.37 for $24,000 paid in, which is $17,103.37 of interest. With $10,000 to start and $100 a month for 10 years at the same rate, you end up with $31,998.32 for $22,000 paid in. With yearly contributions, $1,200 a year for 10 years at 5% gives $15,093.47. The chart shows the growing share of interest over the years.
The rule of 72, monthly compounding and the limits of this simulation
The rule of 72 gives a quick estimate of doubling time: divide 72 by the interest rate. At 6%, 72 / 6 = 12 years, against an exact value of 11.9 years; at 4%, 18 years against 17.67 years. Monthly compounding earns slightly more than annual compounding: 5% a year compounded monthly is equivalent to 5.116% a year (the APY), and $10,000 grows to $16,470.09 in 10 years instead of $16,288.95. This simulation does not take taxes, fees or inflation into account: with 2% inflation, a 5% return only increases your purchasing power by about 3% a year. And past returns do not guarantee future results.
Frequently Asked Questions
What is the compound interest formula?
Final balance = P × (1 + r)^n, where P is the starting amount, r the rate per period and n the number of periods. With contributions C at the end of each period, add C × ((1 + r)^n − 1) / r.
How much will $10,000 earn at 5% for 10 years?
With interest compounded annually, $10,000 grows to $16,288.95, which is $6,288.95 of interest. Compounded monthly, it grows to $16,470.09. Before taxes, fees and inflation.
What is the rule of 72?
A shortcut to estimate how long it takes to double your money: divide 72 by the annual interest rate. At 8%, it takes about 72 / 8 = 9 years; the exact value is 9.01 years.
Monthly or annual compounding: which earns more?
At the same annual rate, monthly compounding earns slightly more, because interest starts earning interest sooner. At 5%, it is equivalent to an annual rate of 5.116%.
Does the calculator include taxes and inflation?
No. The results are gross, before taxes, management fees and inflation. It's an estimate only: future investment returns are not guaranteed.