Inside Your FinanceBook the Intensive
Compound interest

Compound interest calculator: see what happens when you leave it alone.

Enter what you have invested today, anything you add each month, the return you expect and how many years you'll leave it untouched. The chart shows how much of your final balance is growth rather than your own contributions.

Compound growthNet worthInvestment goalDebt payoff

Compound growth calculator

See what your money could become if you leave it untouched — a starting amount, a monthly contribution, or both. Illustrative only.

Contributions are added at the end of each month and compounded monthly at the annual rate ÷ 12. Nothing is withdrawn. Real returns vary.

Value in year 25
$1,338,378
10.3× everything you put in.
Growth from compounding
$1,208,378
On top of the $130,000 you contributed.
If you never touch it
START $100,000
Year 0Year 25

How to use this calculator

  1. Enter the amount you already have invested.
  2. Add what you plan to invest each month — or leave it at $0 for a pure lump sum.
  3. Set the expected annual return. Start conservative, then run it again with a higher number to see the range.
  4. Choose the number of years you plan to leave the money untouched, then read the chart from left to right.

What this assumes

  • Returns compound at a steady rate and monthly contributions are added at the end of each month. Real markets rise and fall, so treat the curve as a trajectory, not a forecast.
  • No taxes, fees or inflation are applied. Fees and inflation both reduce your real result.
  • Set the monthly contribution to $0 if you only want to see a lump sum grow on its own.

Common questions

How does compound interest actually work?

Your returns earn returns. In year one you earn a return on your original amount. In year two you earn a return on the original amount plus last year's growth. Repeat that for 20 or 30 years and most of your final balance is growth rather than the money you put in.

What return rate should I use?

A broad, diversified stock index has historically averaged roughly 7 to 10 percent a year before inflation over long periods. Past returns are not a promise. I usually model a conservative number and an optimistic one so you can see the range instead of one perfect-looking line.

What happens if I don't touch the money?

That's the whole point of this calculator. Withdrawing early removes the balance that would have compounded for the rest of the timeline, so the cost is much bigger than the amount you take out.

Is this calculator free?

Yes. No signup, no email, nothing stored. Change the numbers as many times as you want.

Other free calculators

Need help turning the numbers into a plan?

The calculator shows you the numbers. The Financial Clarity Intensive helps you decide what to do with them.

View the Financial Clarity Intensive→