About this tool
Compound interest pays interest on the interest already earned, so growth accelerates the longer money stays invested. This calculator projects a starting balance plus regular monthly contributions at any annual rate, with monthly, daily, quarterly, or yearly compounding, and shows the year-by-year split between what you put in and what the growth added.
The most useful experiment is time: $300 a month at 7% is $156,000 after 20 years but $366,000 after 30, and most of the second figure is interest. Try a few horizons and contribution levels to see what a small monthly change does over decades.
How to use it
- Enter what you start with and what you will add each month.
- Set the yearly return and the number of years.
- Read the final balance and how much of it is interest rather than your own money.
Questions
What return should I assume?
A savings account: whatever APY it currently pays, 4 to 5% in recent years. A broad stock index fund: about 10% per year over long periods before inflation, roughly 7% after. Bonds: 3 to 5%. Use the after-inflation figure if you want the result in today's money.
Does compounding frequency matter much?
Less than people expect. At 5%, $10,000 compounded daily rather than monthly earns about $1 more per year. Contribution amount and time dominate everything else.
Is anything I type sent anywhere?
No. The calculator runs entirely in your browser; nothing is stored or transmitted. Refresh the page and it starts clean.
Is it free, are files uploaded, which browsers work: the general questions are answered once for the whole site.