A quick mental-math shortcut for how fast money doubles at a given return.
The Rule of 72 is a mental-math shortcut for how long an investment takes to double: divide 72 by the annual rate of return and you get the approximate number of years. This calculator applies it and shows the exact figure alongside.
The precise doubling time comes from logarithms, but 72 divides cleanly by many common rates (2, 3, 4, 6, 8, 9, 12) and lands very close to the exact answer across the range of returns most investors see. At 6% it estimates 12 years (exact ≈ 11.9); at 8%, 9 years (exact ≈ 9.0). The approximation is at its best in the mid-single-digit to low-double-digit range, and drifts a little at very high or very low rates.
The rule works both ways. To find the return needed to double in a set time, divide 72 by the years: to double in 8 years you need about 9% a year. It also makes the cost of inflation vivid, at 3% inflation, prices double in roughly 24 years, halving your money's purchasing power. As a quick sanity check on any 'double your money' claim, dividing 72 by the promised timeline reveals the return being implied.
A shortcut for doubling time: 72 ÷ annual return ≈ years to double. At 8% that's about 9 years. It's accurate for typical investment returns.
Very, for mid-single-digit to low-double-digit rates. It lands within a fraction of a year of the exact logarithmic answer across that common range.
Yes, divide 72 by the inflation rate to see how long until prices double. At 3%, that's about 24 years, which halves your money's purchasing power.
See the exact formula and a worked example on our methodology page.