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Rule of 72 Calculator

The Rule of 72 is a quick mental shortcut for doubling time. Enter a rate to see the estimate beside the exact figure.

Your numbers

Result

Estimate for information only, not professional advice. Last reviewed .

How we calculate it

Rule of 72: years to double ≈ 72 ÷ annual rate in percent. At 8%, that is 9 years.

The exact figure comes from ln(2) ÷ ln(1 + rate). The Rule of 114 does the same for tripling: 114 ÷ rate.

Assumptions and limits

  • The rule is most accurate for rates between about 6% and 10%.
  • It assumes a constant rate with annual compounding and ignores taxes, fees and inflation.

Frequently asked questions

Why 72?

72 has many divisors (2, 3, 4, 6, 8, 9, 12), which makes mental math easy, and it approximates ln(2) × 100 well for typical rates.

How accurate is it?

At 8% the rule gives 9.00 years and the exact answer is 9.01. At 2% it gives 36 years versus an exact 35.0, so it drifts at very low or very high rates.

Can I use it for inflation?

Yes. At 3% inflation prices double in about 24 years, so purchasing power halves in the same time.

Related calculators

Worked example (a fixed example using the starting numbers, it does not change as you type)

Inputs: Annual growth rate (%) = 7.

  • Years to double (Rule of 72) 10.3
  • Exact years to double 10.24
  • Years to triple (Rule of 114) 16.3

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Last reviewed: . Formulas and assumptions are described on our methodology page. For information only; not professional advice.