Rule of 72 Calculator
Estimates doubling time with the rule of 72 and shows the exact figure next to it.
Exact answer
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By the rule of 72 —
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The formula
years ≈ 72 ÷ rate exact years = ln 2 ÷ ln(1 + r)
The exact doubling time comes from solving (1 + r)^t = 2. 72 is a convenient stand-in that divides easily and is close across common rates.
What to watch out for
It works for anything growing at a steady rate, and in reverse too: at 3% inflation, prices double in about 24 years, so money loses half its buying power.
Frequently Asked Questions
What is the rule of 72?
Divide 72 by the annual rate to estimate the years to double. At 8% that is 9 years; the exact answer is 9.01.
How long does money take to double at 5%?
About 14.2 years exactly, or 14.4 by the rule of 72.