Rule of 72 Calculator
A back-of-envelope rule for doubling time. Use the compound calculator above for the exact figure.
Updated May 2026, using HMRC 2026/27 rates and current ONS / gov.uk figures.
72 ÷ annual return = years to double. Useful as a fast mental shortcut. The exact formula uses log(2) ÷ log(1+r) which is what compound interest above computes.
Worked examples
72 ÷ 6% = 12 years to double
72 ÷ 9% = 8 years
72 ÷ 3% = 24 years
Frequently asked questions
Is it accurate?
Within 5% for rates between 5% and 12%. Less accurate at extremes.
Why 72?
Because 72 = ln(2) × 100 / 0.693 ≈ the constant that makes the approximation work for typical interest rates.