Compound interest calculator
Project how a balance grows over time with regular contributions.
Future balance
$0
This runs entirely in your browser. Nothing you enter is uploaded.
How to use this calculator
- Enter your starting amount and any monthly contribution.
- Set an expected annual rate and the number of years.
- Click Calculate to see the projected balance and how much of it is interest.
The formula
Interest is compounded monthly. The starting amount grows by the monthly rate, and each contribution grows for the months that remain.
monthly rate = annual rate / 12
balance = principal x (1 + r)^n + contribution x (((1 + r)^n - 1) / r)
A real example
Starting with 1,000 dollars and adding 200 a month at a 7 percent annual rate for 20 years grows to far more than the cash you put in. Most of the final balance is interest earned on earlier interest, which is the point of compounding: time does the heavy lifting.
Common questions
Is the rate guaranteed?
No. Real returns vary year to year. Use a conservative rate to get a realistic, not best-case, projection.
Does it account for inflation or taxes?
No. The figure is a nominal balance before inflation and taxes. Treat it as a planning estimate.