The rule of 72: how long it takes money to double
Divide 72 by the interest rate to estimate how many years money takes to double. How accurate the shortcut is, and how it works for inflation and debt.
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Tools for this job: Compound Interest Calculator · Percentage Calculator
How long does money take to double at 6% a year? You could work through compound interest, or you can divide 72 by 6 and get 12 years. That shortcut is the rule of 72, and it is accurate enough for most everyday decisions.
How to use it
Years to double ≈ 72 ÷ annual rate (as a plain number, so 6 for 6%).
It also works backwards: to double your money in 9 years, you need about 72 ÷ 9 = 8% a year.
How accurate it is
| Rate | Rule of 72 | Exact |
|---|---|---|
| 2% | 36 years | 35.0 years |
| 4% | 18 | 17.7 |
| 6% | 12 | 11.9 |
| 8% | 9 | 9.0 |
| 10% | 7.2 | 7.3 |
| 12% | 6 | 6.1 |
| 18% | 4 | 4.2 |
| 24% | 3 | 3.2 |
The exact figures assume yearly compounding. Between about 4% and 12% the rule is within a few months. At very low or very high rates it drifts, but it stays a useful first estimate.
Why 72?
The exact doubling time is the natural logarithm of 2, about 0.693, divided by the logarithm of (1 + rate). For small rates that is close to 69.3 ÷ rate. 72 is used instead because it gives better answers at typical rates, and because it divides neatly by 2, 3, 4, 6, 8, 9 and 12. Some people use 69 or 70 for continuously compounded or very low rates.
Inflation halves your money too
The rule works for anything growing at a steady rate, including prices. At 3% inflation, prices double in about 24 years, which means cash kept under a mattress loses half its buying power in that time. At 6% inflation, it takes 12 years.
And debt doubles
Unpaid debt grows the same way. A credit card balance at 24% interest, with nothing paid off, doubles in about three years. Seeing that number is often more persuasive than the interest rate itself.
What fees do to doubling
Small annual fees look harmless until you run them through the rule. An investment growing at 7% a year doubles in about 72 ÷ 7 = 10.3 years. Take off a 1% annual fee and it grows at 6%, doubling in about 12 years. Over 30 years that is roughly three doublings against two and a half: the money ends up about 7.6 times larger instead of about 5.7 times.
Doubling more than once
At 8%, money doubles in about 9 years, so in 18 years it is four times as much and in 27 years eight times. This is why starting early matters so much: the last doubling adds more than all the previous ones together.
When you need the real number
For regular contributions, different compounding periods or exact balances, use the Compound Interest Calculator. It shows the final balance, the interest earned and the effective annual rate. The guide to compound interest explained covers how compounding frequency changes the result.