What Is the Rule of 72?
The Rule of 72 is a shortcut for estimating how long it takes an investment to double: divide 72 by the annual return rate. At 8% a year, money roughly doubles in about 9 years.
Grows to ~2×
$20K
in ~9 years at 8%
What is the Rule of 72?
The Rule of 72 is a piece of mental math. Take the number 72 and divide it by an annual return rate, and you get a rough estimate of how many years it takes for an amount to double.
At an 8% return, 72 ÷ 8 = 9, so money roughly doubles in about 9 years. At 6%, it's 72 ÷ 6 = 12 years. At 12%, it's 72 ÷ 12 = 6 years.
It works because doubling is really a compounding question, and 72 happens to be close to the exact math for the range of returns most long-term investors deal with. It trades a little precision for a lot of speed.
Years to double ≈ 72 ÷ annual return %. It's an estimate, not an exact answer — but it's close enough to do in your head.
Why the Rule of 72 matters
Instant intuition
Turn any return rate into a doubling time in seconds, without a calculator or spreadsheet.
See compounding's speed
A small change in return has a large effect on doubling time — 6% doubles in 12 years, 12% in just 6.
Works for inflation too
Divide 72 by an inflation rate to estimate how fast prices double and purchasing power halves.
Sanity-check claims
If someone implies money will double in 3 years, the rule tells you that needs about a 24% annual return — a useful reality check.
See doubling in action
Adjust the return rate and watch how the doubling time changes — a higher rate reaches 2× far sooner.
Final value
$19,990
Total invested
$10,000
Interest earned
$9,990
The formula
Divide 72 by the annual return, expressed as a whole number (use 8 for 8%, not 0.08). For example, at 8% a year: 72 ÷ 8 = 9 years to double.
Years to Double ≈ 72 ÷ Annual Return Rate (%)Where:
- 72 = a fixed constant that approximates the exact doubling math
- Annual Return Rate = the yearly growth rate, as a whole number (e.g. 8 for 8%)
Real-world example
Suppose you invest $10,000 at an assumed 8% annual return. Using the Rule of 72, 72 ÷ 8 = 9, so you'd expect it to double to roughly $20,000 in about nine years.
The exact compound math gives about $19,990 after nine years — within a few dollars of doubling. That's how close the shortcut gets in the range most long-term investors care about.
Real returns are never this smooth year to year. The rule estimates the average doubling time under a steady-return assumption, not a guarantee of any single outcome.
- Total invested
- $10,000
- Interest earned
- $9,990
- Final value
- $19,990
Common mistakes
Using a decimal instead of a whole number
Divide 72 by 8, not by 0.08. Using the decimal gives an answer that's off by a factor of 100.
Treating it as exact
The rule is an approximation. It's most accurate around 6–10% and drifts at very high or very low rates.
Forgetting it assumes steady compounding
Real returns bounce around. The doubling time is an average under a constant-rate assumption, not a promise.
Confusing it with the Rule of 70 or 69.3
Some prefer 70 or 69.3 for continuous compounding. 72 is popular because it divides cleanly by many common rates.
Frequently asked questions
What is the Rule of 72?
The Rule of 72 is a shortcut that estimates how many years it takes an investment to double: divide 72 by the annual return rate. At 8% a year, money doubles in roughly nine years.
How accurate is the Rule of 72?
It's very close for returns of about 6–10%, usually within a fraction of a year of the exact answer. Accuracy drifts at unusually high or low rates.
Why 72 and not another number?
72 is close to the exact doubling math for typical returns and divides cleanly by 2, 3, 4, 6, 8, 9, and 12, which makes the mental math easy.
Can I use the Rule of 72 for inflation?
Yes. Dividing 72 by an inflation rate estimates how fast prices double and purchasing power halves. At 3% inflation, prices roughly double in about 24 years.
What return do I need to double my money in 10 years?
Rearranging the rule, 72 ÷ 10 ≈ 7.2%, so you'd need about a 7.2% annual return to double in ten years under a steady-return assumption.
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