The Rule of 72: How Fast Will Your Money Double?
Divide 72 by your annual return and you get the years to double your money. At 8%, that's 9 years. It's a back-of-envelope trick that reveals the brutal logic of compounding — and of fees.
Don't have time? Here's what you need to know:
- 1Divide 72 by your annual return to estimate years to double: at 8%, that's 9 years (72 ÷ 8).
- 2A higher return multiplies doublings: over 36 years, 8% doubles four times vs. twice at 4%.
- 3Applied to 3% inflation, prices double in 24 years — a quick warning against long-term cash.
- 4The rule assumes a steady return and is most accurate between roughly 6% and 10%.
The Rule in 30 Seconds
The Rule of 72 is a mental-math shortcut for estimating how long an investment takes to double. You divide 72 by the annual rate of return, and the answer is the approximate number of years to double your money. At an 8% return, 72 divided by 8 equals 9, so money doubles in about 9 years. At 6%, it takes about 12 years; at 9%, about 8.
It works because doubling is governed by exponential growth, and 72 happens to be a number that divides cleanly into many common rates while staying close to the mathematically exact figure. The rule is an approximation, not a formula, but it is accurate enough to do in your head and precise enough to be genuinely useful for quick planning.
How Long Money Takes to Double at Different Rates
The power of the rule is how vividly it shows the difference a few percentage points make. The table below applies 72 to a range of returns. Notice that the jump from a 4% return to an 8% return does not just halve the doubling time once — it changes how many times your money doubles across a lifetime, which is where the real divergence in outcomes comes from.
Over a 36-year horizon, money growing at 8% doubles four times (36 ÷ 9) — turning $10,000 into roughly $160,000. The same money at 4% doubles only twice (36 ÷ 18), reaching about $40,000. A return that is twice as high does not produce twice the wealth; it produces several times more, because each doubling builds on a larger base.
| Annual return | 72 ÷ rate | Years to double |
|---|---|---|
| 4% | 72 ÷ 4 | ~18 years |
| 6% | 72 ÷ 6 | ~12 years |
| 8% | 72 ÷ 8 | ~9 years |
| 10% | 72 ÷ 10 | ~7.2 years |
| 12% | 72 ÷ 12 | ~6 years |
Running the Rule in Reverse: Fees and Inflation
The Rule of 72 is just as revealing applied to things that shrink your money. Apply it to inflation and you learn how fast prices double: at 3% inflation, 72 ÷ 3 = 24 years for prices to double and for cash to lose half its purchasing power. This single calculation explains why holding cash for decades is so quietly destructive.
Apply it to fees and the case for low costs becomes visceral. A 1% annual fee may sound small, but think of it as a 1% drag that compounds against you. Over the decades it takes your money to double several times, that recurring fee silently claims a meaningful share of the doublings you would otherwise keep — which is the entire reason low-cost funds like VOO at 0.03% beat higher-fee alternatives so reliably over time.
Tip: Use the rule on inflation, not just returns: at 3%, prices double in 24 years. That's the number that should make you nervous about long-term cash.
Ready to invest? Open an IBKR account in 10 minutes and get free stock. $0 commissions on US ETFs • Fractional shares from $1 • 150+ global markets.
Where the Rule of 72 Breaks Down
The rule is an approximation, and it is most accurate for returns in the middle of the normal range — roughly 6% to 10%. At very high rates it drifts: for returns around 20% and above, dividing by 69.3 or even using a 'Rule of 70' or '73' gets closer to the exact answer. For everyday investing returns, though, 72 is the most convenient and is accurate to within a fraction of a year.
The bigger limitation is that the rule assumes a single, steady rate of return, which real markets never deliver. Actual stock returns are volatile and lumpy, so the rule tells you the doubling time of an average, not a guarantee for any specific stretch. Treat it as a quick intuition-builder for the power of compounding — not as a precise forecast of when your particular portfolio will double.
Important: The Rule of 72 assumes a steady return. Real markets are volatile, so it estimates the doubling time of an average, not a date you can count on.
Frequently Asked Questions
What is the Rule of 72?
The Rule of 72 is a mental-math shortcut for estimating how long an investment takes to double. You divide 72 by the annual rate of return, and the result is the approximate number of years to double. At an 8% return, 72 divided by 8 is 9, so money doubles in about 9 years.
How accurate is the Rule of 72?
It is very accurate for typical investment returns of roughly 6% to 10%, usually within a fraction of a year of the exact answer. It drifts at very high rates — above about 20%, using 69.3 or 70 gets closer. For everyday planning around stock and bond returns, 72 is both convenient and reliably close.
Can the Rule of 72 be used for inflation?
Yes, and it is one of the most useful applications. Dividing 72 by the inflation rate tells you how fast prices double and cash loses half its purchasing power. At 3% inflation, that is 24 years; at 6%, only about 12 years. It is a quick way to see why long-term cash is risky.
Why is the number 72 used instead of another number?
The mathematically exact constant for continuous doubling is about 69.3, but 72 is used because it divides cleanly by many common rates (2, 3, 4, 6, 8, 9, 12) while staying close to the true figure. That makes it ideal for quick mental math without a calculator across the typical range of returns.
Further Reading
Free Tools
Alex Harrington
CFA Level II Candidate, Finance & Economics
Alex Harrington is an independent ETF researcher and personal finance writer with over 8 years of experience analyzing exchange-traded funds. A CFA Level II candidate with a background in economics, Alex has reviewed 800+ ETFs and helped thousands of beginners build their first investment portfolios through clear, jargon-free education.
This content is for educational purposes only and does not constitute financial advice. Past performance does not guarantee future results. Consult a licensed financial advisor before making investment decisions.