Compound Interest Calculator
Start with a lump sum, add the same amount every month, and see what it grows to. The table shows each year so you can watch growth overtake your own contributions.
| Your contributions | — |
| Growth earned | — |
| Growth share of total | — |
| Years to double (rule of 72) | — |
Year by year
| Year | Contributed | Growth | Balance |
|---|
The formula
The annual return is turned into an equivalent monthly rate, r = (1 + annual)^(1/12) − 1, so 8% a year really ends as 8% after twelve months. Each month the balance grows by r and then your contribution is added. Contributions are made at the end of each month.
Why the later years matter most
In the early years almost all of the balance is money you put in. Growth is earned on growth, so the curve bends upward late. With the defaults above, growth passes your contributions somewhere in the second half of the period. Stopping or withdrawing early cuts off exactly those years.
Want real prices instead of a fixed rate? The growth simulator replays actual market history for any ticker.
A worked example
With the defaults, $5,000 to start, $400 a month and 8% a year for 25 years, you put in $125,000 of your own money and end with roughly $398,000. Growth makes up about two thirds of the final balance. Cut the period to 15 years and you put in $77,000 but end near $151,000, so the last ten years add far more than the extra $48,000 you contribute in them. Raising the rate from 8% to 10% over 25 years adds about $150,000. Small differences in return or time matter more than they look.
Common questions
What annual return should I use?
The S&P 500 has returned roughly 10% a year with dividends over very long periods, but with big swings and stretches of a decade or more that were much worse. Many people plan with 6% to 8% to leave room for bad sequences, fees and taxes. If you want to see real history instead of a smooth rate, try the What If calculator.
Should I adjust for inflation?
The result here is in future dollars. To see it in today's buying power, subtract an inflation estimate from the return, for example use 5% instead of 8% if you expect about 3% inflation. The balance will look smaller, but it will be closer to what it can actually buy.
Monthly or yearly compounding: does it matter?
Very little when the annual rate is converted properly, which this calculator does. The bigger effect comes from when you contribute. Money added at the start of each year has a few more months to grow than money added in December.
What is the rule of 72?
Divide 72 by the annual return in percent to estimate how many years it takes to double. At 8%, about 9 years; at 6%, about 12. It is a shortcut that works well for returns between roughly 4% and 12%.