Rule of 72 Calculator
Use the Rule of 72 to estimate how long an investment takes to double — and the return you would need to double it in a set time.
How it works
Years to double ≈ 72 ÷ annual return% · Return needed ≈ 72 ÷ years
The Rule of 72 is a shortcut for compound growth: divide 72 by the annual return to estimate the years it takes money to double, or divide 72 by the years you have to find the return you would need. It is an approximation of the exact formula (ln 2 ÷ ln(1 + rate)), and it is closest at returns around 8%. Use it for quick intuition, not precise planning.
Worked example
At an 8% annual return, 72 ÷ 8 = 9 years to double — and the exact math agrees at about 9.0 years. To double in 10 years instead, you would need roughly 72 ÷ 10 = 7.2% a year.
Frequently asked questions
Why 72 and not another number?
The true math uses the natural logarithm of 2 (about 0.693). Multiplying by 100 gives 69.3, but 72 divides cleanly by 2, 3, 4, 6, 8, 9 and 12, which makes the mental arithmetic easy while staying accurate near typical returns.
How accurate is the Rule of 72?
Very close for returns between about 5% and 12%. At very low or very high rates it drifts — at 2% it slightly underestimates the time, and at 25%+ it overestimates. For precision, use the exact doubling-time figure shown above.
Does it work for inflation and debt too?
Yes. At 3% inflation, prices double in about 24 years (72 ÷ 3). On a 24% credit card, an unpaid balance can double in about 3 years — the rule cuts both ways.
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This is an educational estimate, not financial or tax advice. Confirm figures with a professional.