Compound Interest Calculator
Results update as you type. Works for regular monthly investing and for a one-off lump sum.
Show year-by-year table
How it works
Every month your contribution is added to the balance, and interest is credited at the chosen frequency. Invest 500 a month at 6% and after 10 years your 60,000 of contributions grows to about 82,300; after 20 years, 120,000 of contributions becomes about 232,000. The second decade earns far more than the first, which is why starting early often matters more than the amount.
For bank deposits and savings plans, use the savings interest calculator. To work out a return percentage, try the percentage calculator or the stock return calculator.
Frequently asked questions
What is the difference between compound and simple interest?
Simple interest is paid only on your original money. Compound interest is added to the balance, so the interest itself starts earning interest. Put 10,000 away at 5% for 20 years and simple interest gives you 20,000, while annual compounding gives you about 26,533. The longer the period, the bigger the gap.
What is the rule of 72?
Divide 72 by the annual return to estimate how many years it takes for money to double. At 6% that is about 12 years; at 8%, about 9 years. When you enter a starting amount with no monthly contributions, this calculator also shows the exact doubling time.
How are monthly contributions handled?
Each contribution is added at the start of the month, and interest is credited at the frequency you choose (monthly, quarterly or yearly) using the annual rate divided by that frequency.
How does the tax field work?
If you enter a tax rate, that share of each interest credit is removed before it is reinvested. Leave it at 0% for tax-free or tax-deferred accounts such as many retirement accounts, or when tax is only due when you sell.
This assumes the same return every year. Real investment returns go up and down, and you can lose money.