The Rule of 72 is the most useful piece of financial math you can hold in your head. Divide 72 by an annual growth rate, and you get the approximate number of years for the principal to double.
The Rule
Years to double ≈ 72 / annual rate (in percent)
- 4% → 18 years
- 6% → 12 years
- 8% → 9 years
- 10% → 7.2 years
- 12% → 6 years
Why 72?
72 is chosen because it has a lot of small divisors (1, 2, 3, 4, 6, 8, 9, 12), making mental division easy. The exact mathematical answer is ln(2) / ln(1+r) ≈ 0.693 / r — and 72 is a clean approximation.
How Accurate Is It?
It's most accurate between 5% and 10% rates. Outside that band, the actual doubling time drifts:
- At 2%, real doubling = 35 years; Rule of 72 says 36. Off by 1.
- At 8%, real doubling = 9.0 years; Rule of 72 says 9. Off by 0.
- At 15%, real doubling = 4.96 years; Rule of 72 says 4.8. Off by 0.2.
For most personal-finance use cases this is plenty good.
A More Accurate Variation
For continuously compounded growth, the Rule of 69.3 is exact: ln(2) × 100 ≈ 69.3. For daily compounding, 70 is closer. For annual compounding at typical rates, 72 wins.
Practical Uses
- Quick retirement math: "At 7%, my savings double every ~10 years."
- Mortgage cost intuition: "At 6% interest, a 30-year mortgage means the lender's money doubles roughly twice during the loan."
- Inflation impact: "At 3% inflation, prices double every 24 years."
Verify with the Tools
The rule is an approximation. For real decisions, use our Compound Interest Calculator or CAGR Calculator — they give exact answers in seconds.
Bottom Line
72 / rate = doubling years. It's good enough for back-of-envelope work and a great way to internalize compound growth.
Frequently Asked Questions
What is the Rule of 72 used for?
The Rule of 72 is a mental-math shortcut for estimating how many years it takes an investment to double in value: divide 72 by the annual growth rate expressed as a percentage. For example, at a 6% annual return, an investment would double in roughly 72 ÷ 6 = 12 years. It's meant for quick estimation, not a precise financial calculation.
How accurate is the Rule of 72?
It's most accurate for annual rates between roughly 5% and 10%, where the estimate is typically off by less than a year compared to the exact calculation. Outside that range, at very low or very high rates, the approximation drifts slightly further from the true doubling time, though it generally remains close enough for everyday personal-finance estimates. For an exact figure, a compound interest or CAGR calculator provides a precise answer.
Can the Rule of 72 be used for things other than investment returns?
Yes — the same math applies to any percentage-based growth rate, including inflation and loan interest. For example, at 3% annual inflation, prices would be expected to roughly double every 72 ÷ 3 = 24 years, which is a useful way to intuitively grasp the long-term erosion of purchasing power. It can similarly be used to reason about how quickly a lender's money effectively doubles over the life of an interest-bearing loan.
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