How accurate is the Rule of 72?

The Rule of 72 is remarkably accurate for interest rates between 5% and 15%, typically within a few months of the exact doubling time. At very low or very high rates, the approximation drifts, and using 69.3 or 70 as the numerator may yield a closer estimate.

Can I use the Rule of 72 for things other than investments?

Yes. It applies to any quantity that grows at a constant percentage rate. Common examples include population growth, GDP growth, inflation, and bacterial growth. Any fixed percentage growth rate can be plugged into the formula.

Does the Rule of 72 account for taxes and fees?

Not automatically. You should use the net return after taxes and fees as your input rate. For example, if your gross return is 10% but fees and taxes consume 2%, enter 8% to get a more realistic estimate of doubling time.

How to Use the Rule of 72

3 min readInvesting

The Rule of 72 is a mental-math shortcut that tells you approximately how many years it takes for an investment to double at a fixed annual rate of return. It is one of the most useful back-of-the-envelope tools in finance, and once you learn it, you will find yourself using it in conversations, meetings, and quick comparisons every day.

Quick Steps

1
Identify the annual rate of return
Determine the expected or historical annual rate of return for your investment, expressed as a percentage.
2
Divide 72 by the rate
Perform the division: 72 divided by the annual rate gives you the approximate number of years to double.
3
Enter the rate in the calculator
Type the rate into the Rule of 72 tool for an instant result along with exact doubling time for comparison.
4
Compare scenarios
Try different rates to see how small changes in return dramatically affect doubling time.

Rule of 72

Estimate how long it takes to double your investment

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The Basic Formula

Years to Double = 72 / Annual Rate of Return (%)

Examples:
  At  6% annual return: 72 / 6  = 12 years to double
  At  8% annual return: 72 / 8  =  9 years to double
  At 12% annual return: 72 / 12 =  6 years to double

Why the Rule of 72 Works

The Rule of 72 is derived from the compound interest formula. Mathematically, the exact number is closer to 69.3 (the natural log of 2 times 100), but 72 is used because it is easily divisible by many common interest rates such as 2, 3, 4, 6, 8, 9, and 12. This divisibility makes mental arithmetic straightforward. The rule is most accurate for rates between roughly 5% and 15%; outside that range, the approximation becomes less precise.

Practical Applications

Estimating how long it takes for retirement savings to double at a given expected return.
Comparing investment options quickly without a calculator.
Understanding the impact of fees: a 2% annual fee on a fund earning 8% effectively reduces the doubling time from 9 years to about 12 years.
Gauging the erosion of purchasing power from inflation (e.g., at 3% inflation, prices double roughly every 24 years).
Teaching financial literacy concepts in an accessible way.

Variations: Rule of 69.3 and Rule of 70

For continuously compounded interest, 69.3 is the mathematically exact divisor. Some practitioners prefer the Rule of 70 as a compromise that balances accuracy and ease of mental calculation. In practice the difference is small: for an 8% return, the Rule of 72 predicts 9.0 years, the Rule of 70 predicts 8.75 years, and the exact answer is 9.01 years. Use whichever number is easiest for the math at hand.

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Frequently Asked Questions

How accurate is the Rule of 72?: The Rule of 72 is remarkably accurate for interest rates between 5% and 15%, typically within a few months of the exact doubling time. At very low or very high rates, the approximation drifts, and using 69.3 or 70 as the numerator may yield a closer estimate.
Can I use the Rule of 72 for things other than investments?: Yes. It applies to any quantity that grows at a constant percentage rate. Common examples include population growth, GDP growth, inflation, and bacterial growth. Any fixed percentage growth rate can be plugged into the formula.
Does the Rule of 72 account for taxes and fees?: Not automatically. You should use the net return after taxes and fees as your input rate. For example, if your gross return is 10% but fees and taxes consume 2%, enter 8% to get a more realistic estimate of doubling time.