Compound Interest Calculator
CalculateProject how an investment or savings balance grows with compound interest and regular monthly contributions, and see how much of the final total is interest rather than deposits.
Runs entirely in your browser. Nothing is uploaded.
Balance after 20 years
$170,619
You put in
$70,000
Interest earned
$100,619
Growth over time
A projection at a fixed rate, not a forecast. Real returns vary year to year, and this does not account for tax, fees, or inflation.
See savings grow over time
Compound interest is the part of saving that is genuinely hard to picture: the balance does not climb in a straight line, because each year's growth earns growth of its own. Put in a starting amount, what you add each month, and a rate, and this shows the year-by-year curve — with the portion you contributed separated from the portion the interest produced, which is usually the more persuasive of the two numbers.
How to calculate compound interest
- 1
Enter your starting balance
The amount you already have invested, or zero if you are starting from nothing.
- 2
Add your monthly contribution
What you plan to add each month. Regular contributions usually matter more to the final total than the rate does.
- 3
Set the rate, term, and compounding
Enter the expected annual return and how many years, then choose monthly, quarterly, or annual compounding.
Good to know
- Contributions are treated as an ordinary annuity, added at the end of each period, and converted to your chosen compounding frequency.
- Compounding frequency matters far less than people expect: at 7% over 20 years, monthly versus annual changes the total by only a couple of percent.
- Results are nominal. To think in today's money, enter a real return — roughly your expected return minus inflation.
- A fixed-rate projection, not a forecast. Real returns vary year to year, and a poor sequence early on changes the outcome substantially.
What people use it for
- Seeing what a monthly retirement contribution could grow into over decades
- Comparing saving a lump sum now against contributing steadily over time
- Working out what rate or contribution you would need to reach a target
Frequently asked questions
How are contributions handled?+
Monthly contributions are treated as an ordinary annuity — added at the end of each period — and converted to whatever compounding frequency you choose. The chart separates what you put in from what the interest added.
Does compounding frequency matter much?+
Less than most people expect. At a 7% return, monthly versus annual compounding changes a 20-year total by only a couple of percent. Contribution size and time invested matter far more.
Does this account for inflation or tax?+
No. Results are in nominal terms. To think in today's money, enter a real rate of return — roughly your expected return minus inflation.
Is this a prediction?+
No. It projects a single fixed rate compounded forward. Real returns vary year to year, and a bad sequence early on produces a very different outcome from the same average return.
Is my data sent to a server?+
No. Calculating happens entirely in your browser using built-in web APIs. Nothing is uploaded, stored, or logged.
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