The fastest mental-math trick in investing. Enter a return to find how long your money takes to double, or enter a target number of years to find the rate you'd need — checked against the exact answer.
Answer first: divide 72 by your annual return and you get the number of years it takes your money to double. This Rule of 72 calculator runs that shortcut both ways — find the years to double from a rate, or the rate you'd need to double in a set number of years — and checks it against the exact logarithmic answer.
Years to double ≈ 72 ÷ Rate. Rate to double ≈ 72 ÷ Years.
Example: at an 8% return, money doubles in 72 ÷ 8 = 9 years. To double in 6 years you'd need 72 ÷ 6 = 12%. The exact answers (using logarithms) are 9.01 years and 12.25% — close enough to do in your head.
The Rule of 72 is the single best mental-math tool in investing because it makes compounding intuitive. At 7% — roughly the long-run real return of the stock market — your money doubles about every decade. Knowing that lets you sanity-check any "get rich" claim instantly: if someone promises to double your money in two years, they're implying a 36% annual return, which should set off alarm bells.
The true doubling time is ln(2) ÷ ln(1 + rate), and ln(2) ≈ 0.693, or 69.3%. So the most accurate divisor is about 69.3 — but 72 is chosen because it's close and divides cleanly by 2, 3, 4, 6, 8, 9 and 12, making the mental math effortless. The approximation is most accurate for rates around 6–10% and drifts a little at the extremes.
The same idea extends: the Rule of 114 estimates time to triple, and the Rule of 144 estimates time to quadruple. This calculator also shows the quadruple time directly. The takeaway never changes — small differences in your annual return compound into huge differences in how fast your wealth multiplies, which is exactly why fees and rate matter so much.
Reality check: The Rule of 72 is an approximation that assumes a single constant compounding rate. Real returns are volatile and never smooth, so treat doubling times as rough guides, not promises. This is an educational calculator, not financial advice — verify your own numbers and see the U.S. SEC at investor.gov.
See doubling play out on the compound interest calculator, learn the idea in what is compound growth, and find your real rate with the annualized return calculator.
Last updated 21 June 2026 · Written by Mustafa Bilgic. Educational only — not financial advice.
The Rule of 72 is a shortcut for estimating how long an investment takes to double: divide 72 by your annual percentage return. At 8% a year, money doubles in about 72 ÷ 8 = 9 years. It works in reverse too, to find the rate needed to double in a set time.
Very, for typical rates. It's most accurate around 6%–10% and drifts slightly at extreme rates. At 8% it estimates 9.0 years versus an exact 9.01. For quick mental math and sanity checks it's more than precise enough.
The mathematically exact divisor is about 69.3 (from the natural log of 2). But 72 is chosen because it's close and divides evenly by 2, 3, 4, 6, 8, 9 and 12, which makes the arithmetic easy to do in your head.
At a 7% annual return, the Rule of 72 gives 72 ÷ 7 ≈ 10.3 years. Since 7% is roughly the long-run real return of the U.S. stock market, a useful rule of thumb is that invested money tends to double about every decade.