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Rule of 72 Calculator

How long to double my money?

How it is calculated

The Rule of 72 is a shortcut for how long money takes to double at a fixed annual growth rate: years to double ≈ 72 ÷ rate, where the rate is in percent per year (enter 6 for 6%). The tool also gives a rough tripling time with the matching constant, years to triple ≈ 114 ÷ rate. Both are approximations to compound growth, and the exact doubling time is ln 2 / ln(1 + r). They work best for rates around 6–10% and are less accurate for very low or very high rates. The rate must be greater than zero. Results are shown with two decimals, as years. Inflation and taxes are not included.

Worked example

An investment grows at 6% per year. Years to double: 72 / 6 = 12.00. Years to triple: 114 / 6 = 19.00. The exact figure for doubling at 6% is ln 2 / ln 1.06 = 11.90 years, so the shortcut is within a few weeks. At 12% the rule gives 72 / 12 = 6 years, while the exact value is 6.12.

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Common mistakes

  • Entering the rate as a decimal such as 0.06 instead of 6, which would give 1,200 years.
  • Applying it to a changing or irregular return; it assumes a constant annual rate.
  • Using it for an investment that pays simple interest rather than compounding.
  • Ignoring inflation and fees; the doubling in nominal terms is faster than in real purchasing power.

Sources

About this calculator

This calculator applies the Rule of 72, a mental-math shortcut that estimates how many years it takes an investment to double by dividing 72 by the annual rate of return — at 8% growth, for instance, 72 ÷ 8 gives roughly 9 years to double, without needing to solve the exact compound interest formula.

It's a favorite of investors and financial educators precisely because it's fast and close enough for real decisions: comparing how a 4% savings account stacks up against a 7% index fund, or getting an intuitive sense of what a country's inflation rate is doing to purchasing power over time (the same shortcut works in reverse to estimate how fast money loses half its value). It won't replace an exact calculation for a legal document, but for a quick gut-check on compounding, it's the tool people reach for first.

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Comments (1)

  • Editorial team

    A useful twist is to enter the real rate instead of the headline one. If an account pays 8% and inflation runs at 3%, typing 5 gives 72 ÷ 5 = 14.4 years for your purchasing power to double, which is a more honest planning figure than 9 years for the nominal balance. Taxes and fees can be subtracted the same way. What rate are you weighing, and is it before or after inflation? Post the figures you compared.

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