Compound interest calculator
with monthly contributions.
Enter what you have, what you add each month and the return you expect. See the final balance, how much of it is interest, and what it's worth in today's money.
Year-by-year table
| Year | Contributed | Interest | Balance | In today's money |
|---|
How it's calculated.
Month by month, with the contribution added after each month's interest.
Each month the balance grows by the monthly equivalent of your annual rate, then that month's contribution is added. Over n months that gives the standard future-value formula:
FV = P × (1 + i)n + C × ((1 + i)n − 1) ÷ i
P is the initial amount, C the monthly contribution and i the monthly rate. With monthly compounding, i is the annual rate divided by 12. With quarterly, yearly or daily compounding, i is the monthly rate that gives the same effective yearly return, so contributions still arrive every month.
In today's money divides the final balance by (1 + inflation)years: what that amount would buy at today's prices.
- Contributions arrive at the end of each month. Money added at the start would earn one extra month of interest each time.
- The rate never changes. Markets don't work that way: treat the result as one scenario and try a lower rate too.
- No taxes or fees. Both lower the final balance. Subtracting yearly fees from the rate is a fair rough adjustment.
200 a month, for 20 years.
These are the calculator's starting values, so the chart above shows the same run.
Start
10,000
Every month
200
Return
7% a year
Interest adds 20,714 in the first 10 years and 65,859 in the next 10, on exactly the same 200 a month. The second decade earns more than three times the first, because by then the interest is being paid on interest. That is why time in the market does more of the work than the size of each contribution.
Frequently asked questions.
What is compound interest?
Interest earned on both the money you put in and the interest it has already earned. Because each year's interest is added to the balance, the next year's interest is calculated on a bigger number, so growth speeds up over time.
What is the difference between monthly and yearly compounding?
How often interest is added to the balance. At the same nominal rate, more frequent compounding ends slightly higher: 7% compounded monthly is an effective 7.23% a year.
What interest rate should I use?
It depends on where the money is. A savings account or deposit pays its advertised rate. For a diversified stock portfolio, try several rates rather than one, because past returns don't guarantee future ones.
Does the calculator include taxes and fees?
No. Both reduce the result. As a rough adjustment, subtract yearly fees from the rate: for example, use 6.5% instead of 7% for a fund that charges 0.5%.
Is the calculator free? Do I need an account?
Yes, it's free and needs no signup. The calculation runs in your browser, and the numbers you type aren't stored or sent anywhere.
From projection
to your real balance.
A calculator assumes a steady rate. Moneydy tracks what actually happens: savings, stocks and crypto with prices that update themselves, and savings goals that show how far along you are. Try the live demo, no signup.