TOOLWatch compound interest work
Enter a starting amount, an optional monthly contribution, an interest rate and a time horizon, and this calculator shows the future value of your savings — split into what you put in versus what interest added. It's the clearest way to see why starting early matters so much.
How it's calculated
Each monthly deposit is added and then compounds for the rest of the term, so later contributions earn less interest than earlier ones — another reason time in the market beats timing it.
The takeaways
- Time is the biggest lever — decades of compounding dwarf the starting amount.
- Contributions add up — small, regular deposits compound into large sums.
- Rate matters — even one or two percent changes the ending balance dramatically over 20+ years.
Estimates for planning only. Real returns vary and aren't guaranteed, and this doesn't account for taxes, fees or inflation. Not financial advice.
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original amount and the interest already added. Because each period's interest earns interest of its own, savings grow faster over time than with simple interest.
How does compounding frequency affect growth?
The more often interest compounds — daily versus monthly versus yearly — the more you earn, because interest starts earning interest sooner. The difference is small over a year but adds up over decades.
Does this include regular contributions?
Yes. You can add a recurring monthly contribution, and the calculator compounds both your starting balance and each contribution to show the total future value and how much of it is interest.