Estimate how long it takes to double your investment at a fixed annual rate of return using the classic Rule of 72.
The Rule of 72 is a quick mental-math shortcut for estimating how many years it takes an investment to double in value at a fixed annual compound interest rate. You simply divide 72 by the annual rate of return. It's not perfectly exact, but it is remarkably close for typical interest rates between 4% and 15%.
Rule of 72: Years to double ≈ 72 / rate (%)
Exact compound-interest formula (for comparison): Years = ln(2) / ln(1 + rate/100)
1. Enter the annual interest rate you expect to earn (as a percentage).
2. Optionally enter a starting investment amount to see the doubled value.
3. Click Calculate to see both the quick Rule of 72 estimate and the exact answer.
At an 8% annual return:
Rule of 72 estimate = 72 / 8 = 9 years
Exact formula = ln(2) / ln(1.08) ≈ 9.01 years
The two answers are almost identical — this is why the Rule of 72 is so widely used for quick estimates.
A shortcut estimating years to double an investment at a given rate: years ≈ 72 ÷ interest rate.
Good for rates roughly 6-10%, slightly less accurate outside that range.
Yes — the same shortcut estimates years for purchasing power to halve at a given inflation rate.