Compound Interest Calculator
The eighth wonder of the world, quantified.
Final balance:
Total you contributed:
Interest earned:
Growth over time
How to use this calculator
Enter your starting amount, monthly contribution, annual return and the number of years, then press Calculate. The final balance, what you contributed, and the interest earned are shown with a growth curve.
Worked example
Start with $5,000, add $200 a month and earn 8% a year, and in 15 years you have roughly $85,000 - of which you put in only $41,000. Compounding supplied the other $44,000. Delay the same plan by five years and the ending balance drops by tens of thousands, because you lose the years where growth grows on growth.
What the result means
The tool projects your balance from a starting amount, monthly contributions and an assumed annual return. Time is the multiplier: contributions matter, but the number of years they compound matters just as much.
How compounding actually works
- Each year you earn a return not only on your deposits but on all prior returns.
- Monthly contributions get their own compounding runway, so steady early saving beats a big late lump sum.
- The assumed return is a long-run expectation - real results swing widely year to year.
Frequently asked questions
What return should I use?
6-8% is a common long-run planning range for a diversified stock portfolio after inflation. Use a lower, more conservative number if you cannot tolerate the swings.
Use the tool twice - once now and once with a five-year-later start date - to see what waiting really costs. That comparison, more than any rate assumption, is what convinces people to start today: the compounding years you skip are the ones you can never buy back.