What is the Rule of 72?

Years to double ≈ 72 ÷ annual interest rate (in percent). At 9% a year, money doubles in about 8 years.

How accurate is the Rule of 72?

Within a few percent for rates between about 4% and 15%, and nearly exact near 8%. It slightly overstates the doubling time at low rates (36 years at 2% versus 35.0 exact) and understates it at high rates (3.6 years at 20% versus 3.8).

What is the difference between the rules of 72, 70 and 69.3?

They use different constants. 69.3 is exact for continuous compounding, 70 is easy for mental arithmetic, 72 fits annual compounding at typical interest rates best.

Can I use the Rule of 72 for inflation?

Yes. Dividing 72 by the inflation rate estimates how many years it takes for prices to double.

Rule of 72 Calculator and Formula

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Rule of 72 Calculator

The Rule of 72 is a mental shortcut for compound growth: divide 72 by the annual interest rate in percent and you get the approximate number of years it takes for money to double. At 8% a year, 72 ÷ 8 = 9 years; at 6%, 12 years. It also works in reverse, so to double your money in 10 years you need about 7.2% a year.

Formula

Years to double ≈ 72interest rate in % exact: t = ln 2ln(1 + r)

Where

72: a constant chosen because it is close to 100 × ln 2 ≈ 69.3 and divisible by many numbers
r: annual rate of return (annual compounding)
t: years for the money to double

Calculator

Annual rate of return (%)

Result and step-by-step solution

Years to double (Rule of 72)

Exact doubling time at 8% a year: 9.0065 years (the rule is off by 0.006 years).

Divide 72 by the rate in percent
728 = 9 years
Exact answer
t = ln 2ln(1 + 0.08) = 0.6931470.076961 = 9.0065 years
Other rules of thumb
Rule of 70 8.75 years
Rule of 69.3 (continuous compounding) 8.6625 years
Rule of 72 9 years

Rule of 70	8.75 years
Rule of 69.3 (continuous compounding)	8.6625 years
Rule of 72	9 years

How to use the formula

The exact doubling time with annual compounding is ln 2 ÷ ln(1 + r). The number 72 is used because it is close to 100 × ln 2 = 69.3, adjusted upward to compensate for annual rather than continuous compounding at typical rates, and because it divides evenly by 2, 3, 4, 6, 8, 9 and 12. The rule is most accurate around 8%; the calculator shows the size of the error for your rate.

The page also reports the Rule of 70, often used for population and GDP growth, and the Rule of 69.3, which is exact for continuous compounding. For a precise answer with monthly or daily compounding, use the doubling time formula. The same rule tells you how fast inflation halves purchasing power: at 4% inflation, prices double in about 18 years.

Frequently asked questions

What is the Rule of 72?: Years to double ≈ 72 ÷ annual interest rate (in percent). At 9% a year, money doubles in about 8 years.
How accurate is the Rule of 72?: Within a few percent for rates between about 4% and 15%, and nearly exact near 8%. It slightly overstates the doubling time at low rates (36 years at 2% versus 35.0 exact) and understates it at high rates (3.6 years at 20% versus 3.8).
What is the difference between the rules of 72, 70 and 69.3?: They use different constants. 69.3 is exact for continuous compounding, 70 is easy for mental arithmetic, 72 fits annual compounding at typical interest rates best.
Can I use the Rule of 72 for inflation?: Yes. Dividing 72 by the inflation rate estimates how many years it takes for prices to double.

Last reviewed: September 26, 2026. Calculations run in your browser and were checked against numpy-financial, SciPy and published spreadsheet examples.