Compound Interest Calculator
Project a balance forward with monthly contributions and monthly compounding. See the final balance, what you put in, and what interest actually earned.
Year-by-year
Compounding is monthly; the table below shows the balance at the end of each year (capped at 50 rows).
| Year | Balance | Contributed | Interest earned |
|---|
How to read this number
The final balance is only partly your own money. As the time horizon stretches out, a growing share of the ending balance comes from interest earned on interest, not from new contributions. That's the "time matters more than amount" effect Learn readers ask about.
Formula & assumptions
Common questions
How does compound interest work?
Compound interest is growth on growth: each period's return is calculated on the original principal plus everything it has already earned, so the balance grows faster the longer it compounds.
What is a realistic rate to use for stock market compounding?
There is no single right number, since future returns are never guaranteed. Many long-term planning exercises use a range of assumptions rather than one fixed rate, and past averages are not a promise of future results.
Does monthly contribution amount matter more than the interest rate?
Both matter, and their relative impact depends on your time horizon and starting balance. Over shorter periods, contributions tend to drive more of the balance; over longer periods, the rate compounds and its effect grows.
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