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Rule of 72 Calculator

Estimate how many years it takes money to double at a given return, and compare with the exact answer.

Result

Enter your values and select Calculate to see the result.

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How the Rule of 72 Calculator works

The rule of 72 is a mental shortcut: divide 72 by the yearly return to estimate how many years money takes to double — or divide 72 by the years to find the return you need. The calculator also shows the exact answer with yearly compounding, so you can see how close the shortcut is.

Formulas

Rule of 72

years to double ≈ 72 ÷ rate %

Exact

years = ln 2 ÷ ln(1 + rate)

Examples

An 8% return

72 ÷ 8 = 9 years; the exact answer is 9.01 years — the rule is almost perfect here.

Inflation at 2%

72 ÷ 2 = 36 years for prices to double, against an exact 35.00 — at low rates the rule of 70 (35 years) is closer.

Frequently asked questions

Why 72?

The exact constant for continuous compounding is about 69.3, but 72 is divisible by 2, 3, 4, 6, 8, 9 and 12, which makes mental maths easy, and it's more accurate for common yearly rates around 6–10%.

How accurate is it at high rates?

It gets worse: at 24% the rule says 3 years but the exact answer is 3.22. Use the exact figure for big rates.

Can I use it for debt?

Yes — unpaid debt at 12% doubles in about 6 years, which is a vivid reason to pay it off.

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