The rule of 72 estimates how many years it takes for an investment to double: divide 72 by the annual return. At 6%, money doubles in about 12 years. At 9%, about 8 years. It is a mental shortcut, accurate to within a few months for returns between roughly 4% and 15%.
The formula
Years to double ≈ 72 ÷ annual return (in percent)
| Annual return | Rule of 72 | Exact doubling time |
|---|---|---|
| 3% | 24 years | 23.4 years |
| 6% | 12 years | 11.9 years |
| 9% | 8 years | 8.0 years |
| 12% | 6 years | 6.1 years |
Below 4% the rule slightly overestimates; above 15% it underestimates. For everyday decisions it is close enough.
Why 72 and not 100?
Doubling requires a 100% gain, so you might expect 100 ÷ rate. But compounding means each year’s growth is calculated on a larger balance, so the doubling happens sooner. 72 is the number that makes the approximation work across normal return ranges — it is close to 100 × ln(2), which is about 69.3, adjusted for the way the approximation behaves at typical rates.
Three uses that matter
1. Retirement
At 7%, money doubles about every 10 years. $50,000 at 35 becomes roughly $100,000 at 45, $200,000 at 55, and $400,000 at 65 — before adding a single contribution. That is why starting a decade earlier usually beats contributing more later.
2. Debt
The rule works against you too. A credit card at 24% doubles an unpaid balance in about three years (72 ÷ 24 = 3). Minimum payments mostly cover interest, which is why balances feel frozen.
3. Inflation
At 3% inflation, prices double in about 24 years. Your cash does not fall — it just buys half as much. This is the argument for holding cash only as an emergency fund and not as long-term savings.
A worked comparison
Two people invest $10,000 once.
- Ana starts at 25 at 7%: by 65 the money has doubled about four times, reaching roughly $150,000.
- Ben starts at 45 at the same 7%: two doublings, roughly $38,000.
Same amount, same return, four times the outcome — because of time, not skill.
What the rule does not do
- It ignores volatility. A 7% average return with big swings does not double as smoothly as the rule suggests.
- It ignores fees and tax. A 1% fee turns 7% into 6%, and 12 years into 12.6. See expense ratios.
- It assumes a constant rate. Markets do not deliver one.
Common mistakes
- Using a hoped-for return. 12% is not a plan; it is a hope. Use a conservative rate for planning.
- Forgetting inflation. Doubling in 12 years at 6% is only about 1.5× in real terms if inflation runs at 3%.
- Applying it to shrinking balances. On a loan you are repaying, the balance does not compound upward the same way.
- Treating it as exact. It is an estimate; use it to compare options, not to plan to the dollar.
How to use it in a decision
- Take the return you realistically expect, after fees.
- Divide 72 by it to get the doubling time.
- Count how many doublings fit before you need the money.
- Compare that with the cost of waiting.
Frequently asked questions
Is the rule of 72 accurate?
Within a few months for rates between about 4% and 15%. Outside that range the error grows.
Does it work for inflation?
Yes. Divide 72 by the inflation rate to see how long until prices double.
Can I use it for monthly returns?
Not directly. Convert to an annual rate first, or use 72 with the annualised figure.
Reviewed 22 October 2026. Educational content, not financial advice. See our disclaimer.
