Compound Interest Calculator With Monthly Contributions
Compound interest means your returns start earning returns. Add a starting amount, a monthly contribution and an annual rate to see the growth.
How it works
With monthly compounding at monthly rate r for n months: FV = P(1+r)n + PMT × ((1+r)n − 1) ÷ r, where P is the starting amount and PMT the monthly contribution (added at each month end).
Example: $10,000 starting, $500 a month, 7% annual, 20 years: future value $300,850.72. You invested $130,000, so interest earned is $170,850.72 — more than you put in.
Frequently asked questions
Is compounding monthly or yearly here?
Monthly: the annual rate is divided by 12 and applied each month, which is how most savings accounts and investment illustrations work.
Why does time matter more than the amount?
Because growth is exponential: doubling the years roughly quadruples the interest, while doubling the contribution only doubles it.
Are taxes and inflation included?
No. This is a gross illustration — real returns are reduced by taxes, fees and inflation.
What if the rate is 0%?
Then the future value is just what you put in: starting amount plus monthly contribution times months.