Rule of 72 calculator
Estimate how long it takes to double your money at any rate of return using the classic mental-math shortcut.
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Rule of 72
What it measures
A mental math shortcut for estimating how long an investment takes to double in value at a fixed annual rate.
When to use it
Use it for quick checks. At a 12% annual return, money doubles in about 6 years (72 / 12).
Watch out for
It is an approximation and becomes less accurate at very high rates.
Rule of 72
How many years it takes for money to double.
How it works
Years to double ≈ 72 / annual return %Compound growth means a fixed percentage return doubles your money in a fixed number of years, and 72 happens to be an excellent numerator for the mental arithmetic because it divides cleanly by 2, 3, 4, 6, 8, 9, and 12. The rule converts any growth rate into an intuitive doubling clock, and it works both ways: it also tells you how quickly inflation halves your purchasing power.
Worked example
At 8%, money doubles in about 72 ÷ 8 = 9 years (the exact answer is 9.01; the rule is nearly perfect in the 4-12% range). At 6% it's 12 years; at 12%, six. Flip it around: 3% inflation halves the value of cash in roughly 24 years, and a 2% account fee quietly eats a doubling every 36 years. Four doublings in a 36-year investing life at 8% turns €25,000 into €400,000, which is the whole argument for starting early, in one line of arithmetic.
Common questions
How does the Rule of 72 work?
Divide 72 by your annual return to estimate the years needed to double your money. At 8%, that's 72 ÷ 8 = 9 years. It works because of the mathematics of compound growth.
How accurate is the Rule of 72?
Very accurate between about 4% and 12%, within a few weeks of the exact answer. At extreme rates it drifts: the true doubling time at 72% isn't exactly one year.
Can I use it for inflation too?
Yes, it works for anything that compounds. At 3% inflation, prices double (and cash halves in purchasing power) roughly every 24 years.
Last reviewed July 2026