Rule of 72 Calculator
Estimate how long it takes for your money to double at a given rate.
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The Rule of 72 is a quick mental math shortcut: divide 72 by your annual return rate to estimate how many years it takes for an investment to double. At 7%, money doubles in about 10.3 years. At 10%, about 7.2 years. This calculator shows both the Rule of 72 estimate and the exact mathematical answer, plus projections at 10, 20 and 30 years.
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Frequently asked questions
Divide 72 by the annual interest rate to get the approximate doubling time. At 6%, money doubles in 72/6 = 12 years. At 8%, it doubles in 72/8 = 9 years. The rule is most accurate for rates between 2% and 15%.
72 is used because it has many small divisors (2, 3, 4, 6, 8, 9, 12) making mental math easy. The mathematically exact number is 69.3 (ln(2) × 100), but 72 gives a better approximation for typical investment rates due to the slight error in the linear approximation.
Yes, the Rule of 72 works for any growth rate. At 3% inflation, prices double in about 24 years. This means a dollar today buys roughly half as much in 24 years.
Very accurate for rates between 2-15%. At 7%, the Rule of 72 gives 10.29 years vs the exact answer of 10.24 years - a difference of less than 1%. At very high or very low rates, the accuracy decreases.
For tripling, use the Rule of 115 (divide 115 by the rate). For quadrupling, double the doubling time (or use the Rule of 144). At 7%: doubles in ~10 years, triples in ~16 years, quadruples in ~21 years.