The Rule of 72: Double Your Money Math in Seconds
A mental-math shortcut that tells you how long it takes money to double at any interest rate, and how fast inflation halves it.
Some financial math needs a calculator. This one needs only your head. The Rule of 72 is a brilliantly simple shortcut for one of the most important questions in investing: how long will it take my money to double?
The rule in one line
Years to double ≈ 72 ÷ annual rate of return
That's it. Earning 8% a year? Your money doubles in about 72 ÷ 8 = 9 years. Earning 6%? About 12 years. It turns an abstract return into a concrete timeline you can actually picture.
Doubling time at different returns
| Annual return | Years to double (72 ÷ rate) |
|---|---|
| 4% | ~18 years |
| 6% | ~12 years |
| 8% | ~9 years |
| 10% | ~7.2 years |
| 12% | ~6 years |
Notice how much a few percentage points matter. The difference between 6% and 10% isn't small, it's the difference between doubling your money in 12 years versus about 7. Over a lifetime, that compounds into an enormous gap.
It's a quirk of the compound-interest math: 72 divides neatly by many common rates (2, 3, 4, 6, 8, 9, 12) and lands close to the precise answer. That's why it beat similar "Rule of 70" and "Rule of 69" versions for everyday use.
Use it in reverse: inflation
The rule works just as well for things you don't want doubling. Divide 72 by the inflation rate to see how fast prices double, and your money's buying power halves. At 3% inflation, prices double in about 24 years. At 6%, just 12. It's a vivid reminder of why cash left idle quietly loses value, as we explain in our guide to inflation.
Weighing a safe 3% account against an investment averaging 8%? The Rule of 72 says one doubles your money in ~24 years, the other in ~9. Seen that way, the long-term cost of playing it too safe becomes obvious.
The limits
The Rule of 72 is an approximation. It's most accurate for rates between about 5% and 12%; at very high or very low rates it drifts. And real returns aren't smooth, markets rise and fall. Treat it as a fast sanity check, not a precise forecast.
See the exact growth curve. Enter a rate and time period to watch your money compound, and double, year by year.
Open the Compound Interest Calculator →The bottom line
The Rule of 72 turns interest rates into timelines you can grasp in seconds. Use it to compare investments, to understand the real cost of inflation, and to remind yourself why even a small bump in returns, sustained over time, changes everything.
Frequently asked questions
What is the Rule of 72?
It's a shortcut for estimating how long an investment takes to double: divide 72 by the annual rate of return. For example, at 8% a year, money doubles in roughly 72 ÷ 8 = 9 years.
How accurate is the Rule of 72?
It's a close approximation for typical rates between about 5% and 12%. Outside that range it drifts a little, but it remains a handy mental estimate.
Can the Rule of 72 measure inflation?
Yes. Dividing 72 by the inflation rate tells you how long it takes prices to double, and therefore how fast your money's buying power is being cut in half.