Rule of 72 (doubling your money)
Use the Rule of 72, an investor's mental shortcut, to quickly estimate how many years it takes for an investment to double at a given annual rate of return — or the reverse, the rate needed to double an investment in a given number of years. 100% local calculation.
How it works
The Rule of 72 is a mental math shortcut used by investors to estimate, without a calculator, the time needed to double money invested at compound interest: simply divide 72 by the annual rate of return (in %). This tool provides this quick calculation along with the mathematically exact result, based on a logarithm, so you can compare the two.
Where does the number 72 come from in this rule?
The number 72 is chosen because it divides easily by many common rates (2, 3, 4, 6, 8, 9, 12...) and gives a reasonably accurate approximation for typical investment rates (roughly between 3% and 15%). For very high or very low rates, the gap with the exact calculation becomes more significant.
Why does the exact result differ slightly from the approximation?
The exact formula uses a logarithm (years = ln(2) / ln(1 + rate)), which precisely reflects compound-interest growth. The Rule of 72 is a linear approximation of this curve, convenient to calculate in your head but slightly imprecise, especially outside the range of rates it was designed for.
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