The Rule of 72: How Fast Your Money Doubles
- •72 ÷ annual return (%) = years to double
- •6% takes about 12 years, 9% about 8 years
- •Used on inflation, it shows when money loses half its value
What is the Rule of 72?
The Rule of 72 is a quick way to estimate, in your head, how many years it takes money to double under compound growth. The math is simple: divide 72 by your annual return in percent. At 8% a year, 72 ÷ 8 = 9, so your money roughly doubles in about 9 years.
Why 72? The exact doubling formula needs logarithms, which are hard to do in your head. 72 is a close approximation that fits especially well in the 6–10% range and divides cleanly, so it has long been used as mental math. It is a tool for getting a feel for an investment horizon without a calculator.
Working through examples
The higher the return, the shorter the doubling time. The table below shows the rough years to double at different annual returns. For instance, $10,000 at 6% becomes about $20,000 in 12 years, then about $40,000 in another 12.
| Annual return | Years to double |
|---|---|
| 3% | ~24 years |
| 6% | ~12 years |
| 8% | ~9 years |
| 9% | ~8 years |
| 12% | ~6 years |
In reverse: when money halves
The Rule of 72 works just as well on rates that eat away value. Divide 72 by the inflation rate to see when your cash loses half its real buying power. At 3% inflation, 72 ÷ 3 = 24, so in about 24 years the same money buys only half of what it does today. That is why idle cash quietly shrinks.
Frequently Asked Questions
Why 72 and not 69?
The most accurate figure is about 69.3. But 72 divides evenly by 2, 3, 4, 6, 8, and 9, making mental math far easier. So convention trades a little accuracy for a number that is simple to divide.
What if returns vary each year?
Plug in an average return for a rough estimate. Because real returns rise and fall, it is not a precise forecast — just a feel for roughly how many years at that pace.
Does it work on losses?
Yes — applied to a shrinking rate, it gives the time to halve. Use it on things that erode value by a steady percentage each year, like inflation or fees, to estimate a half-life.
How do I use it in practice?
It is handy in reverse. If you want to double in 10 years, the return you need is 72 ÷ 10 = about 7.2%. It quickly links a target horizon to the return required, so you can sanity-check whether it is realistic.
Does it apply to simple interest?
No — the Rule of 72 assumes compounding. With simple interest, where interest is paid only on the principal, the doubling time is different. It fits only when interest earns interest and snowballs.