Compound Interest Calculator
See how your money grows over time — and exactly how much of the result is your contributions versus investment growth.
Your plan
$
$
%
20 years
How often interest is credited to your balance — your monthly contribution stays the same.
%
Estimated balance after 20 years
$300,851
You invested$130,000
Investment growth$170,851
Adjusted for 3% annual inflation — in today's dollars
Real value today–
Real contributions–
Real growth–
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.
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.