Rule of 72 Doubling Time Calculator

About this calculator

The Rule of 72 is a mental-math shortcut for estimating how many years it takes an investment to double at a given annual compound rate: divide 72 by the interest rate (as a whole number, so 6 for 6%), and the result is roughly the number of years to doubling. It works because it approximates the exact formula for doubling time under compound interest, ln(2) / ln(1 + r), and 72 is chosen over the mathematically purer 69.3 because it divides evenly by more common interest rates (6, 8, 9, 12), making the arithmetic easier to do in your head.

Investors and financial educators have used the Rule of 72 for centuries — it appears as far back as Luca Pacioli's 1494 mathematics text — as a quick sanity check on compound growth, whether for comparing investment returns, estimating how debt at a given interest rate will grow, or illustrating the power of compounding in a classroom setting without needing a calculator.

This calculator gives you both the Rule of 72 approximation and the exact doubling time from the precise compound-interest formula, so you can see how close the shortcut gets at your specific rate.