Find out how long it takes to double, quadruple, or 8x your money. The Rule of 72 is the simplest way to estimate compound growth — no spreadsheet needed.
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The Rule of 72 is a simple mental math shortcut: divide 72 by your annual return rate to estimate how many years it takes your money to double. For example, at 8% return, 72 / 8 = 9 years to double.
It works remarkably well for rates between 2% and 20%. The formula comes from the natural logarithm of 2 (approximately 0.693) — using 72 instead of 69.3 makes mental division easier with common rates like 6%, 8%, 9%, and 12%.
For higher precision, use the exact formula: Years = ln(2) / ln(1 + r). Or for continuous compounding: Years = ln(2) / r. Our calculator uses the exact formula for accuracy while displaying the Rule of 72 estimate alongside.