The Rule of 72 is the fastest mental-maths trick in personal finance: divide 72 by your annual return rate to find how many years your money takes to double.
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Works in reverse too: 72 ÷ years = required rate. The exact answer is ln(2) / ln(1+r). Rule of 72 is accurate to within ±1 year for rates of 6–15%.
The Rule of 72 is an approximation. For precise planning, use the full compound interest formula. See our full disclaimer.