Rule of 72 Calculator

About this calculator

The Rule of 72 answers one specific question fast: at a given annual compound rate, roughly how many years until an investment doubles? Divide 72 by the interest rate — at 8% annual return, 72 ÷ 8 = 9 years — and you get a close estimate without touching a calculator, because 72 approximates the exact compound-growth relationship ln(2) / ln(1 + r) while dividing evenly by common rates like 6, 8, 9, and 12.

It's a favorite shortcut of investors, financial advisors, and personal-finance educators for comparing how fast different rates of return compound — a 4% savings account versus an 8% stock-index fund, for instance — and it works just as well in reverse, estimating what rate of return you'd need to double your money in a target number of years.

This calculator takes an annual rate of return and returns the estimated number of years to double, using the Rule of 72.