Rule of 72 Calculator
Quickly estimate how long it takes money to double at a given annual growth rate.
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The Rule of 72 is a mental-math shortcut: divide 72 by the annual interest rate to estimate the number of years needed for an investment to double. At 8% growth, money doubles roughly every 9 years (72 ÷ 8). It's an approximation, most accurate for rates between about 6% and 10%; the exact formula uses natural logarithms.
Where the number 72 comes from
The exact doubling time is ln(2) ÷ ln(1+r), which for small r is approximately 0.693/r. Multiplying by 100 for a percentage rate gives about 69.3, but 72 divides evenly by more whole numbers (2, 3, 4, 6, 8, 9, 12), which is why it became the standard shortcut over the more precise 69.3.
Frequently asked questions
Does the Rule of 72 work for inflation too?
Yes, it works for any compounding rate. At 3% inflation, prices double roughly every 24 years (72 ÷ 3). The same math applies to debt growing at a fixed interest rate.