Rule of 72 Calculator
Work out how many years it takes to double your money at a given rate, with the exact formula alongside the shortcut.
Enter a number above — the answer updates as you type.
- Rule of 72
- — years
- Exact doubling time
- — years
- Rule of 72 error
- —
- Doubled amount
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- Rule of 70 / 69.3
- — / —
- Doublings in your horizon
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- Value at the end of the horizon
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- Rule of 72 rate needed
- —%
- Exact rate needed
- —%
| Doubling | Rule of 72 (years) | Exact (years) | Balance |
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Educational estimate only, not financial advice. Real returns vary, and the figures here ignore tax, fees and inflation unless you enter an inflation rate on purpose.
How it works
The Rule of 72 is a mental-maths shortcut for compound growth: divide 72 by the annual
percentage rate and you get roughly the number of years it takes for money to double.
At 8% a year that is 72 ÷ 8 = 9 years. The exact answer, for interest
compounded once a year, comes from t = ln(2) ÷ ln(1 + r) — 9.01 years at 8% —
so the shortcut is off by less than a week over nearly a decade. This page shows both, plus
the Rule of 70 and the Rule of 69.3 (the continuous-compounding version, since
ln(2) ≈ 0.693), so you can see how close each variant lands.
72 is chosen because it divides cleanly by 2, 3, 4, 6, 8, 9 and 12, and because it is most accurate around 6–10% a year. Below about 3% the Rule of 70 is closer; for continuously compounded rates use 69.3. The rule works in reverse too: 72 divided by the number of years you have gives the return you would need, and read as an inflation rate it tells you how long prices take to double — or your purchasing power to halve. Everything is calculated in your browser with plain JavaScript; no numbers are uploaded, and there are no ads or sign-ups.
Frequently asked questions
What is the Rule of 72?
The Rule of 72 is a mental-maths shortcut for compound growth: divide 72 by the annual percentage rate and you get roughly the number of years it takes for an amount to double. At 8% a year, 72 ÷ 8 = 9 years.
How accurate is the Rule of 72?
Very accurate between about 6% and 10%. The exact doubling time for interest compounded once a year is ln(2) ÷ ln(1 + r), which gives 9.01 years at 8% versus the shortcut's 9.00, an error of 0.07%. The gap widens at very low or very high rates, which is why this calculator shows both numbers side by side.
When should I use the Rule of 70 or 69.3 instead?
Use 70 for low rates (below roughly 3%), where it lands closer, and 69.3 for continuously compounded rates, since ln(2) ≈ 0.693. The number 72 is popular mainly because it divides cleanly by 2, 3, 4, 6, 8, 9 and 12, making the division easy in your head.