Compound Interest Calculator with Monthly Contributions
See how a starting balance and steady monthly investing grow with compound returns, and how much of the result is growth rather than your own money.
Horizontal axis: years from today.
The compound interest formula
We compound monthly: FV = P(1+i)^n + PMT × ((1+i)^n − 1) ÷ i, where i is the monthly rate equivalent to your annual return, n is the number of months, P your starting balance and PMT your monthly contribution.
Frequently asked questions
How much will $500 a month be worth in 30 years?
At a 7% annual return, investing $500 a month for 30 years grows to about $584.7K, of which only $180,000 is money you put in.
What is the rule of 72?
Divide 72 by your annual return to estimate how many years it takes money to double. At 7% that is about 10 years; at 10%, about 7.
What return should I assume?
The S&P 500 has returned roughly 10% a year nominal, or about 7% after inflation, over the long run. Use 7% for a nominal estimate in future dollars, or 5% for a conservative after-inflation estimate.
More calculators
- FIRE calculator Your FI number and the age you can retire.
- How long until I can retire? Years to retirement from your savings rate.
- Coast FIRE calculator When you can stop saving and coast.
- Safe withdrawal rate calculator Odds your portfolio outlasts you.
- Dividend income calculator Your future dividend income.
For education only, not financial advice. Returns are assumptions, not promises; all figures are in today's dollars.