Rule of 72 Calculator
The Rule of 72 estimates how many years it takes your money to double at a given annual return.
How it's calculated
Divide 72 by your return written as a whole number and you get the rough number of years for money to double. At 8% that is about 9 years. At 6% it is about 12. At 12% it is about 6.
The trick works because compounding is exponential, and 72 happens to be a convenient number that divides cleanly by many common rates. It is meant for quick mental math, not precise planning.
For an exact projection with contributions and a specific time horizon, use the Compound Interest calculator. The Rule of 72 is best for a fast gut check, like seeing how much sooner your money doubles if you can earn 9% instead of 6%.
Assumptions
- An approximation. It is 72 divided by the return percentage, and it is most accurate for rates near 8%.
Last updated: 2026-08-07
These assumptions follow our general methodology.
Frequently asked questions
How accurate is it?
It is a mental-math shortcut. It is very close for returns between roughly 6% and 10% and drifts a little at the extremes.
Can I use it for inflation?
Yes. Divide 72 by the inflation rate to estimate how long until prices double and your money loses half its value. At 3% inflation that is about 24 years.
Where does the number 72 come from?
It is a rounding of the math behind compounding. The exact figure shifts slightly by rate, but 72 is close across the range most investors care about and is easy to divide in your head.