About the Rule of 72 calculator
The rule of 72 is a quick mental shortcut: divide 72 by the annual interest rate to estimate the years needed to double money. This calculator shows the estimate next to the exact figure.
How the Rule of 72 calculator works
Years to double ≈ 72 ÷ rate; exact = ln 2 ÷ ln(1 + rate ÷ 100)
Enter an annual rate compounded yearly. The rule is most accurate around 8% and drifts further from the exact figure at very low or very high rates.
Worked example
- Annual rate
- 8%
Example inputs
- Years to double (exact)
- 9.01 years
- Rule of 72 estimate
- 9 years
- Difference
- −0.01 years
Calculated result
Why use a Rule of 72 calculator?
- Compare rates quickly without a calculator.
- See how much faster money grows at a higher rate.
- Estimate how fast inflation halves purchasing power.
Rule of 72 calculator FAQs
Why 72?
Because 72 is close to the exact constant for typical rates and divides evenly by many numbers, which makes mental arithmetic easy.
Can it be used for inflation?
Yes. Dividing 72 by the inflation rate estimates how many years prices take to double, or money to lose half its purchasing power.
Is there a rule of 70 or 69?
Yes. They suit continuous compounding and lower rates. The exact figure in the calculator needs no rule.
Does it work for monthly compounding?
It is meant for yearly compounding. Monthly compounding doubles money slightly faster.