Compound Interest Calculator

How to use the calculator

1. Enter the amount you start with and how much you add each month.
2. Choose an annual rate. It is an assumption: real returns change every year and can be negative.
3. Choose the number of years and, if you want, an inflation rate to see the result in today's purchasing power.
4. Read the final balance, how much you contributed, how much came from growth, and the table by year. Copy the page address to share your scenario.

The formula

With monthly compounding, the balance after n months is FV = P × (1 + r/12)^n + C × [((1 + r/12)^n − 1) ÷ (r/12)], where P is the starting amount, C the monthly contribution, r the nominal annual rate and n the number of months. Contributions are assumed to be made at the end of each month.

With no contributions it is the classic FV = P × (1 + r/12)^n. At a 0% rate, FV = P + C × n.

The rule of 72

To estimate how long money takes to double, divide 72 by the annual rate in percent. At 6% it takes about 12 years; at 3%, about 24. It is an approximation that works best for rates between roughly 5% and 10%.

Assumptions and limits

• The rate is constant. Real investments rise and fall, and the order of good and bad years changes the outcome.
• Taxes, fees and trading costs are not included. Fees reduce the effective rate.
• Inflation, when entered, is applied as an annual reduction of purchasing power. It does not change the nominal balance.
• Savings accounts, bonds and stocks have very different typical rates. The scenarios (rate ±2 points) show how sensitive the result is.
• Nothing here is a promise or a forecast.

What matters most

Continue with how to save money or how to start investing. To see income from dividends, use the dividend income calculator.

• Time. The last decade adds more than the first because growth builds on a larger balance.
• Regular contributions. Adding every month often matters more than the rate you guess.
• Costs. A 1% fee takes a large share of 30 years of growth.