Some financial questions don’t need a spreadsheet. “How long will it take my money to double?” is one of them, and the Rule of 72 answers it in seconds. Divide 72 by the annual growth rate, expressed as a whole number, and the result is roughly the number of years it takes for a sum to double. At 8% a year, that’s 72 ÷ 8 = 9 years.
It’s a shortcut, not a law of physics — but a remarkably good one across the range of rates people actually deal with. Once it’s in your head, you can size up compound growth on the spot, without reaching for a calculator.
Why it works
The exact time to double at a compounding rate comes from logarithms: divide the natural log of 2 (about 0.693) by the log of one plus the rate. The Rule of 72 is a clean approximation of that messier formula. The number 72 gets used instead of 69 or 70 because it has so many tidy divisors — 2, 3, 4, 6, 8, 9, 12 — which makes the mental math painless.
The Rule of 72 isn’t magic — it’s the doubling-time formula rounded into something you can do in your head, and the rounding is close enough to trust for quick estimates.
Where it’s most accurate
The approximation is sharpest for rates in the middle of the everyday range — roughly 6% to 10% — where it lands within a fraction of a year of the true value. As rates move to the extremes, the estimate drifts a little. At very high rates it slightly understates the doubling time; at very low rates the gap widens too. Some people nudge the numerator toward 70 for low rates and 74 for high ones, but for casual use 72 is fine across the board.
A few quick checks against the exact answer:
- At 6%: the rule says 12 years; the true value is about 11.9. Nearly exact.
- At 9%: the rule says 8 years; the true value is about 8.04. Excellent.
- At 2%: the rule says 36 years; the true value is about 35. Still a useful ballpark.
Quick mental-math examples
The rule shines when you want to feel the impact of a rate without grinding through compounding:
- A hypothetical 8% return. Money doubles about every 9 years, so $25,000 becomes roughly $50,000 in 9 years and $100,000 in 18 — three doublings would take about 27 years.
- 3% inflation. Run the rule in reverse and prices double in about 24 years, which is the same as your money losing half its purchasing power over that time.
- 18% credit-card interest. An unpaid balance can double in just 4 years — a vivid reminder that compounding cuts both ways.
You can also solve it the other direction. If you want your money to double in 6 years, divide 72 by 6 to see you’d need about a 12% annual return — and then judge for yourself how realistic that is. All of these figures are illustrative; actual returns and inflation vary and are never guaranteed.
Its limits
The Rule of 72 assumes a single, steady rate and reinvested growth. Real markets don’t deliver smooth returns, so treat the answer as an order-of-magnitude estimate rather than a forecast. It also ignores taxes and fees, both of which lengthen the true doubling time. And it answers only one narrow question — how long to double — so for anything involving regular contributions or precise targets, reach for a full calculation. As a back-of-the-napkin tool for building intuition about compounding, though, few tricks are more useful.