✍️ By Cristian Meza Venegas · Published Oct 28, 2026 · 6 min read
The Rule in One Sentence
Divide 72 by an annual growth rate, and the answer is roughly the number of years it takes for something to double. At 6% a year, money doubles in about 72 ÷ 6 = 12 years. At 9%, it doubles in about 8 years. At 2%, it takes around 36 years. That is the whole rule — and it is one of the most useful pieces of mental arithmetic in personal finance.
Why It Works
The exact doubling time for a rate r is ln(2) ÷ ln(1 + r). For small rates, ln(1 + r) is very close to r itself, and ln(2) is about 0.693 — so the true "rule" would be the Rule of 69.3. Bankers settled on 72 because it is easy to divide in your head (it is divisible by 2, 3, 4, 6, 8, 9 and 12) and because it happens to be more accurate than 69 for the interest rates people most often deal with, around 6–10%.
| Annual rate | Rule of 72 | Exact doubling time |
|---|---|---|
| 2% | 36.0 years | 35.0 years |
| 4% | 18.0 years | 17.7 years |
| 6% | 12.0 years | 11.9 years |
| 8% | 9.0 years | 9.0 years |
| 10% | 7.2 years | 7.3 years |
| 12% | 6.0 years | 6.1 years |
| 24% | 3.0 years | 3.2 years |
As the table shows, the rule is remarkably close between about 4% and 12% and only drifts at very high rates. For a quick sanity check, that is more than accurate enough.
Using It for Your Savings
The rule turns abstract percentages into a timeline you can picture. Suppose you are 30 and invest $10,000 in a diversified fund that averages 7% a year. That money doubles roughly every 10 years: to about $20,000 at 40, $40,000 at 50, and $80,000 at 60 — without adding another cent. Wait until 40 to invest the same amount and you lose one entire doubling, ending around $40,000 at 60. This is the single clearest illustration of why starting early matters so much.
For precise numbers — including regular monthly contributions — try our compound interest calculator.
Using It in Reverse: Inflation
The same rule tells you how fast inflation halves the value of cash. At 3% inflation, prices double — and the purchasing power of money sitting idle halves — in about 24 years. At 6% inflation, it takes only 12. That is why long-term savings kept entirely in cash quietly lose value, and why retirement plans must account for rising prices. Our inflation calculator shows exactly what a sum of money will be worth after any number of years.
Using It for Debt
The rule works just as well against you. A credit-card balance growing at 24% APR doubles in roughly three years if left unpaid. Seeing debt in those terms makes the case for paying it off quickly very hard to argue with. The debt payoff calculator shows how much faster you can be debt-free by adding even small amounts to your monthly payment.
Using It for Fees
Fees are a negative growth rate. An investment fund charging 1% a year might sound cheap, but over 30 years that 1% compounds into a substantial share of your final balance. Comparing a 7% return with a 6% return using the rule — doubling every 10.3 versus 12 years — shows how one percentage point can mean an entire extra doubling over a long career.
Related Rules: 69, 70 and 114
- Rule of 69.3 — the mathematically exact version for continuously compounded growth.
- Rule of 70 — popular in economics for low rates such as GDP or population growth.
- Rule of 114 — estimates how long it takes money to triple (114 ÷ rate).
- Rule of 144 — estimates how long it takes money to quadruple.
The Takeaway
You do not need a spreadsheet to understand compound growth. Divide 72 by the rate and you instantly know how fast your savings will grow, how fast inflation erodes cash, and how fast unpaid debt can spiral. It is a small piece of arithmetic that can change big decisions.
Disclaimer: This article is for general educational purposes and is not financial advice. Investment returns are not guaranteed and can be negative.
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