Rule of 72 Calculator
Quickly estimate how long it takes to double your money — or what return rate you need to double in a target number of years.
| Rate | Rule of 72 (2x) | Exact (2x) | Rule of 114 (3x) | Exact (3x) | Rule of 144 (4x) | Exact (4x) |
|---|
How the Rule of 72 Works
Divide 72 by the annual interest rate to get the approximate number of years it takes to double your money. At 6%, it takes 72 / 6 = 12 years. At 9%, about 8 years. It's a mental shortcut that gives results surprisingly close to the exact logarithmic calculation.
When It's Accurate vs When It Breaks Down
The Rule of 72 is most accurate for rates between 6% and 10%. At lower rates (2–4%) it slightly overestimates doubling time; at higher rates (15–20%) it increasingly underestimates. The exact formula is ln(2) / ln(1 + r). The table above shows the error at each rate.
Practical Uses
Use it to quickly evaluate investment options, understand how inflation erodes purchasing power (at 3% inflation, purchasing power halves in 24 years), or estimate how long a savings account will take to double. The Rule of 114 and Rule of 144 apply the same concept to tripling and quadrupling money, respectively.