The Rule of 72, Explained
The Rule of 72 estimates how many years it takes to double your money at a given return. The formula, a doubling-time table, worked examples, and when it’s accurate.
The short version
- Years to double ≈ 72 ÷ annual return (as a percent). At 8%, money doubles in about 9 years.
- It works in reverse too: 72 ÷ years gives the return you’d need to double in that time.
- The estimate is most accurate for returns between about 6% and 10%.
- The same trick shows how fast inflation halves your money: 72 ÷ inflation rate.
The Rule of 72 is the most useful piece of mental math in personal finance. It answers a simple question — how long will it take to double my money? — without a spreadsheet or a calculator. You just divide 72 by your annual rate of return, and the answer is roughly the number of years it takes to double.
The formula
There’s only one thing to remember:
Years to double ≈ 72 ÷ annual return
Use the return as a plain number, not a decimal — so 8% is 72 ÷ 8, not 72 ÷ 0.08. The result is the approximate number of years for your money to double.
At an 8% return, 72 ÷ 8 = 9 years to double. At 6%, it’s 72 ÷ 6 = 12 years. The math works because compound growth is exponential, and 72 happens to be a number that’s both close to the true doubling math and easy to divide (it splits cleanly by 2, 3, 4, 6, 8, 9, and 12).
Rule of 72 table
Here’s the doubling time for common annual returns — no math required:
| Annual return | Years to double |
|---|---|
| 2% | 36 years |
| 4% | 18 years |
| 6% | 12 years |
| 8% | 9 years |
| 10% | 7.2 years |
| 12% | 6 years |
| 15% | 4.8 years |
It works backwards, too
Flip the formula and it tells you the return you’d need to double in a set time: 72 ÷ years = required return. Want to double your money in 10 years? You’d need about 72 ÷ 10 = 7.2% a year. Want to double in 6 years? About 12%. This is a fast sanity check on any investment’s promises.
The inflation version
The same rule shows how fast rising prices cut your money’s value in half — just divide 72 by the inflation rate. At 3% inflation, prices double (and a dollar’s buying power halves) in about 24 years. At 6%, it’s only 12. It’s a stark reminder that cash sitting idle isn’t staying still; it’s quietly losing ground.
When the shortcut is (and isn’t) accurate
The Rule of 72 is an approximation, and it’s most accurate for returns between about 6% and 10% — right in the range of typical long-term stock market returns. Outside that band it drifts a little: at very low rates it slightly overestimates the time, and at very high rates it underestimates it. For a more precise answer at extreme rates, some people use 69.3 (the mathematically exact constant for continuous compounding) or 70. For everyday planning, 72 is close enough and far easier to divide.
Try it with your numbers
Enter a return to see the doubling time, or enter a timeframe to see the return you’d need — with the exact compound math alongside the Rule of 72 estimate.
Try the calculator
Rule of 72 CalculatorUse the Rule of 72 to estimate how long an investment takes to double, or the return needed to double in a set number of years.To see the full growth curve — not just the doubling point — run a real compound-interest projection with regular contributions.
Sources
This article is for general education and is not financial, tax, or legal advice. Figures reflect published 2026 IRS and SSA amounts as of the date above; verify current limits with the linked sources or a qualified professional before acting.
About the author
David Miles is the founder of FigureMoney and builds independent, source-backed personal-finance tools across the Modern Site Builders network. Every calculator and guide cites the IRS, SSA, or primary research behind its numbers.
Calculators in this guide
Rule of 72 Calculator
Use the Rule of 72 to estimate how long an investment takes to double, or the return needed to double in a set number of years.
Compound Interest Calculator
Free compound interest calculator. See how savings and investments grow with an initial amount, regular contributions, interest rate, and compounding frequency.
CAGR Calculator
Calculate the compound annual growth rate (CAGR) of an investment from its beginning value, ending value, and the number of years.
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