Compound Interest Calculator

See how your money grows over time — and exactly how much of the result is your contributions versus investment growth.

If you invest $10,000 today and add $500 every month, at a 7% annual return your money could grow to about $300,851 in 20 years. You would contribute $130,000; compounding would add roughly $170,851 — more than half of the final balance. Compound interest works because your returns start earning returns of their own, so growth accelerates over time. Adjust any amount, rate or time horizon below to model your own plan, and see the exact contribution-versus-growth split.

Your plan

Calculations run in your browser. Inputs are not stored.

Initial investment

Monthly contribution

Annual return

Illustrative assumptions only. Adjust the rate for your own model.

Time horizon

20 years

Compound frequency

How often interest is credited to your balance — your monthly contribution stays the same.

Adjust for inflation

Estimated balance after 20 years

$300,851

You invested$130,000

Investment growth$170,851

ContributionsGrowth

Show yearly breakdown

Year	Contributions	Growth	Balance

How this calculation works

The formula

With monthly contributions added at the end of each month, the balance after N months is:

FV = P · (1+i)^N + M · ( ((1+i)^N − 1) / i )

P = initial investment · M = monthly contribution · i = monthly rate (annual rate ÷ 12, adjusted for compound frequency) · N = number of months.

Assumptions

Contributions are added at the end of each month (ordinary annuity).
The annual return is a nominal rate; results are pre-tax and pre-inflation.
Returns are compounded at the frequency you select (default: monthly).
The return rate is assumed constant for the whole period — no market volatility is modeled.
Fees and taxes are not included, so real returns will be lower.
When "Adjust for inflation" is on, the final balance and your contributions are discounted to today's dollars using a constant annual inflation rate.

View the formula →

Educational purposes only. Not financial advice. See the full methodology and disclaimer.

Compound interest, explained

How do you calculate compound interest with monthly contributions?

We compound your balance monthly and add your monthly contribution at the end of each month. The balance after N months is FV = P(1+i)^N + M(((1+i)^N−1)/i), where i is the monthly rate, P the initial investment and M the monthly contribution.

Does the calculator include taxes and inflation?

By default results are nominal, pre-tax and pre-inflation. Turn on Adjust for inflation in the calculator to see your result in today's dollars — it discounts the final balance and your contributions by the inflation rate you choose (default 3%). Taxes and fees are still not included.

Why does compound frequency matter?

More frequent compounding credits interest to your balance more often, so it starts earning its own interest sooner. At the same nominal rate, daily compounding grows slightly faster than annual compounding — try switching the setting above to see the difference.

What's a realistic annual return to use?

Historical stock-market averages are often quoted around 7–10% before inflation (nominal). A 4% rate is a conservative assumption, and 10% is an optimistic long-run stock return. Past performance never guarantees future results.