Rule of 72 Calculator
Estimate how many years it takes to double your money at a given rate of return.
About the Rule of 72 Calculator
Enter a yearly rate of return (or inflation, or interest) to see roughly how many years it takes for an amount to double, with the exact answer and the Rule of 70 and 69.3 versions for comparison.
Divide 72 by the yearly percentage to estimate doubling time. It's most accurate around 6–10%. The same trick works for inflation: at 3% inflation, prices double in about 24 years.
The formula
Years ≈ 72 ÷ rate (%); exact = ln 2 ÷ ln(1 + r)
Worked example
Money growing at 8% a year
- 72 ÷ 8 = 9 years
- Exact: ln 2 ÷ ln 1.08 = 9.01 years
- So 10,000 becomes about 20,000 in 9 years and 40,000 in 18
Things to keep in mind
- An approximation: at rates far from 8% it drifts (at 1% it says 72 years; the exact answer is 69.7).
Questions people ask
Why 72?
It's close to 100 × ln 2 (69.3) adjusted for yearly compounding, and has many divisors.
Does it work for debt?
Yes: at 24% APR, a card balance doubles in about 3 years if unpaid.
What rate doubles money in 10 years?
About 7.2% (72 ÷ 10).