How to use the rule of 72
- Enter an annual interest rate as a percentage.
- Read the years to double, triple and quadruple instantly.
- Use it to compare investments, loans or inflation at a glance.
Why 72, 114 and 144
The number 72 is chosen because it divides cleanly by many common rates and tracks the natural log math closely. For tripling, 114 is the matching shortcut; for quadrupling, 144. All three assume compounding at a steady rate.
A worked example
At 8% a year, money doubles in 72 / 8 = 9 years, triples in 114 / 8 = 14.3 years, and quadruples in 144 / 8 = 18 years. The estimates are close to the exact compound-interest answer and far faster to compute in your head.
Where it breaks down
The rule works best for rates between about 3% and 15%. At very low or very high rates the shortcut drifts from the precise figure, so treat it as a sanity check rather than a contract.