How long until prices double? The rule of 72 and inflation
Divide 72 by the inflation rate and you get, roughly, how many years it takes for money to lose half its buying power.
Divide 72 by the yearly inflation rate and you get a rough answer to "how long until prices double?". The rule of 72 estimates the number of periods needed for doubling by dividing 72 by the percentage rate per period. Because it works for any steady growth, it also gives the time for buying power to halve, which is the same story told from the other side: prices doubling means each pound or dollar buys half as much.
Try it with the numbers in the news. Inflation is the fall in that happens when the general level of prices rises, so each unit of money buys fewer goods and services. As of October 2026 the UK's inflation was 3.5% over the previous 12 months. At 3.5% inflation, one version of the rule (using 70 instead of 72) gives about 70/3.5 = 20 years for the value of a unit of currency to halve. At the Bank of England's 2% target, 72 divided by 2 gives 36 years.
Step 1: Take the yearly rate
For example 3.5% inflation, or 6% interest.
Step 2: Divide 72 by it
72 ÷ 6 = 12. The rule of 70 gives 70 ÷ 3.5 = 20.
Step 3: Read the years
That is roughly how long until prices double, or buying power halves.
The rule is much older than calculators. The text where it was written down presents an example in plain words: at 6 percent a year you divide 72 by 6, which gives 12, and in 12 years the capital is doubled. The author, Luca Pacioli, did not explain why it works, and it is assumed that the rule predates him.
The lesson is not that prices will follow this path, only what a steady rate adds up to. A small yearly percentage sounds harmless, but compounding turns it into something much bigger over a working life.
Try it
When do prices double?
- 100 buys after 20 years
- 50
- Prices double in
- 20.1 yrs
Challenge · Find the rate at which prices double in about 10 years (the slider moves in half-percent steps, so get as close as you can). Does 72 divided by 10 agree?
Quiz me
0/3
Recap
Divide 72 by the yearly rate: that is roughly the number of years until prices double.
💡 A trick to remember it · Seventy-two is the key that opens every 'how long?' door: divide it by the rate and the years come out.
Surprising fact · At 3.5% inflation the rule of 70 gives about 20 years for the value of a unit of currency to halve.
Connects to
- 🛒 What is happening to UK prices, and how do the US and eurozone compare?
- 🧩 Why do prices rise: too much demand, higher costs or too much money?
- 📈 How did a question about compound interest reveal the number e?
- 🪷 Why does something that doubles daily look harmless until the last moment?
- 💵 Why does money lose value over time?
Sources (4)
No source, no claim. Every fact in this lesson (13 claims) cites at least one of these.