A one-second mental shortcut: divide 72 by a rate to see how many years it takes money to double — or to halve.
Direct answer: divide 72 by the annual rate to estimate the years for an amount to double. At 8% a year, money doubles in about 72 ÷ 8 = 9 years. The same trick run on inflation tells you how fast your savings halve.
| Annual rate | Years to double |
|---|---|
| 4% | ~18 years |
| 6% | ~12 years |
| 8% | ~9 years |
| 12% | ~6 years |
Run it on inflation to see the damage: at 15% inflation, idle money loses half its purchasing power in about 72 ÷ 15 ≈ 5 years. That single number explains why doing nothing is a decision — and an expensive one.
See the exact erosion with the Inflation Calculator.
Divide 72 by an annual rate to estimate the years for money to double (at a return) or halve (at inflation).
It is a close approximation for rates roughly between 4% and 15% — perfect for quick mental math.