Rule of 72 Calculator
Quickly estimate how many years it takes to double your money at a given interest rate.
Assumptions used in this calculation
- Approximation: The Rule of 72 is a mental-math shortcut, most accurate for interest rates roughly between 6% and 10%.
About this calculator
The Rule of 72 is a classic finance mental-math shortcut: divide 72 by an annual interest or growth rate, and the result approximates how many years it takes a sum to double under compounding, without solving the actual exponential equation. It's genuinely useful for quick comparisons, is an 8% fund likely to double roughly twice as fast as a 4% one, but relying on the exact number 72 divided in your head is easy to fumble, and the approximation itself degrades outside a moderate rate range. This calculator does the division precisely for whatever rate you enter, so you get an instant, accurate doubling-time estimate for comparing investment options, checking how fast debt grows if left unpaid, or estimating how quickly inflation will erode purchasing power, all without needing to remember or re-derive the shortcut yourself.
Worked example
8% interest
Result: 9 years to double
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