Tools · Rule of 72

How fast does your money double?

The Rule of 72 is the back-of-napkin trick every investor knows: divide 72 by your return to get the years to double. Run it either way below.

Last updated: June 2026

The Rule of 72 is a mental-math shortcut for estimating how many years it takes an investment to double at a given fixed annual rate of return: divide 72 by the interest rate (as a whole number, not a decimal). At 6% annual growth, money doubles in roughly 72 ÷ 6 = 12 years; at 8%, roughly 9 years; at 3%, roughly 24 years.

Why 72 (and not a "cleaner" number)

The rule is an approximation of the exact compound-growth formula, where the true doubling time is ln(2) ÷ ln(1 + r) ≈ 0.693 ÷ r for small r. Using 72 instead of the mathematically purer 69.3 is a deliberate trade-off: 72 divides evenly by more small whole numbers (2, 3, 4, 6, 8, 9, 12), making the mental arithmetic easier, and it happens to track the true formula more closely in the common 6%-10% return range that most long-term investors actually experience.

How accurate it is

The approximation is quite good between roughly 6% and 10% — within a few percent of the exact answer — but drifts further off at very low or very high rates (below about 4% or above about 20%), where the underlying logarithmic curve bends away from the simple linear approximation the rule assumes.

Why it matters

Beyond investment returns, the same shortcut applies to anything compounding — debt growing at a given interest rate, inflation eroding purchasing power (at 3% inflation, prices roughly double every 24 years), or population/economic growth rates. It's most useful as a fast sanity check, not a substitute for a full compound-interest projection with actual contributions, which is why this site also has a dedicated compound interest calculator for precise numbers.

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Doubling time at a glance

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An approximation, most accurate for ~6–10% returns. Education, not financial advice.

Frequently asked questions

What is the Rule of 72?
A shortcut for how long an investment takes to double: divide 72 by the annual return %. At 8% a year, money doubles in ~72 ÷ 8 = 9 years.
How accurate is the Rule of 72?
A close approximation for rates between about 6% and 10%. At very low or very high rates it drifts slightly from the exact figure, but it's accurate enough for quick mental math and planning.
How long to double my money at 7%?
About 10.3 years (72 ÷ 7). At 10% it's ~7.2 years, and at 4% about 18 years — higher returns shorten doubling time dramatically.
When should I use the Rule of 72?
For a fast gut-check on compounding, or to work backward to the return you'd need to double money by a date. For precise projections, use the compound interest calculator.