Divide 72 by the inflation rate to get the doubling time. At 3% — close to the long-run UK and US average — prices double in 24 years, so cash left alone that long buys half as much.
It is the Rule of 72 pointed at the other side of the ledger. Savers apply it to growth and forget it applies to the money itself: nothing on a bank statement ever goes down, so the loss is invisible unless you convert. The scale is easy to underestimate — £100 in 1970 needed £1,557 by 2024 to buy the same things, which is 15.6 times, not the “a bit more” most people guess.
The long-run average describes no actual decade. UK prices rose 242% during the 1970s alone — roughly 13% a year, which halves cash in under six years, not 24. And it can run the other way: US consumer prices *fell* between 1920 and 1940, so $100 in 1920 needed only $70 in 1940 for the same basket. Money gaining value is rare, but a rule that cannot express it is describing a trend rather than a mechanism. CPI is also a basket average, and nobody buys the basket — if your spending is weighted towards housing or energy your own rate has been well above the headline for years.
Estimate with the rule, then check it against the calculator that models it properly.
Open Inflation Calculator →Divide 72 by the inflation rate to get the doubling time. At 3% — close to the long-run UK and US average — prices double in 24 years, so cash left alone that long buys half as much. It is the Rule of 72 pointed at the other side of the ledger.