Estimate doubling time with 72 ÷ rate, and compare it to the exact compound formula.
| Years (rule of 72) | – |
| Years (exact) | – |
Use the rule of 72 as a mental shortcut for how long money takes to double at a constant annual rate. It is an approximation; the exact years use continuous-style compounding via logarithms. The rate must be positive.
Rule-of-72 years = 72 ÷ rate%, where rate is the percent number (8 for 8%). Exact years = ln(2) / ln(1 + rate). Type 8 for 8%.
Rate (%): Annual compound rate as a percent, e.g. 8 for 8%.
Years (rule of 72): The shortcut doubling time.
Years (exact): Doubling time from ln(2) / ln(1 + r).
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