Rule of 72 Calculator

The Rule of 72 is one of the most useful mental shortcuts in finance. It estimates how long it takes for an investment to double in value at a fixed annual rate of return: simply divide 72 by the rate. At 8% a year, money doubles in about 72 ÷ 8 = 9 years; at 6%, it takes roughly 12 years. The same rule works in reverse — divide 72 by the number of years you have, and you get the return you'd need to double in that time. To double in 5 years, you'd need about 72 ÷ 5 = 14.4% a year.

The rule works because compound growth is exponential, and 72 happens to be a convenient approximation of the math (the precise figure derives from natural logarithms, where the true constant is about 69.3). The reason 72 is preferred is that it divides cleanly by 2, 3, 4, 6, 8, 9 and 12, making the arithmetic easy to do in your head. It's most accurate for returns in the 6–10% range; at very high or very low rates the estimate drifts slightly from the exact answer, but it stays close enough for quick decisions.

The Rule of 72 is valuable far beyond investing. It illustrates the power of compounding in a way percentages alone don't: a one-point difference in return can dramatically change how fast your money grows. It also works for anything that compounds, including inflation — at 3% inflation, prices double in about 24 years, which is a stark way to see how rising costs erode purchasing power over a working lifetime. You can even apply it to debt: a credit card charging 24% interest would double an unpaid balance in roughly three years.

For precise figures, use a full compound interest calculation, since the Rule of 72 is an approximation. But as a fast sanity check — "how long until this doubles?" — it's hard to beat, and it makes the abstract idea of compounding tangible.