To estimate how long until prices double, divide 72 by the annual inflation rate. At 3% inflation, prices double in about 72 / 3 = 24 years; at 4% it is roughly 18 years; at 6% about 12 years. It is the same mental-math shortcut investors use for doubling money, just pointed at the cost of living instead of returns.
The one-line method
The Rule of 72 says that any quantity growing at a steady percentage doubles in roughly 72 divided by that percentage, in years. Inflation is exactly that kind of steady percentage growth: prices compound year after year, so the same rule applies. Plug your inflation assumption into the Rule of 72 calculator and the doubling time drops out instantly.
This is a back-of-the-envelope estimate, not a precise forecast. It tells you the order of magnitude of how fast your dollar loses ground, which is usually all you need to make a smart long-term decision.
How fast do prices double at different inflation rates?
Here is the shortcut next to the exact answer (the precise math is in the next section). Notice how close the quick estimate stays across normal inflation levels:
| Annual inflation | Rule of 72 estimate | Exact doubling time |
|---|---|---|
| 2% | 36 years | 35.0 years |
| 2.5% | 28.8 years | 28.1 years |
| 3% | 24 years | 23.4 years |
| 3.5% | 20.6 years | 20.1 years |
| 4% | 18 years | 17.7 years |
| 5% | 14.4 years | 14.2 years |
| 6% | 12 years | 11.9 years |
| 8% | 9 years | 9.0 years |
Across the realistic 2%-to-6% band, the Rule of 72 is within a fraction of a year of the truth. That is why it survives as a kitchen-table tool.
What "prices doubling" actually does to your money
When prices double, your dollar buys half as much. A simple way to feel it: at 3% inflation, the buying power of cash is cut in half about every 24 years.
- A $5 coffee today costs roughly $10 in 24 years (the exact figure at 3% is $10.16).
- A $50 grocery run becomes about $100 in 24 years, and roughly $200 in 48 years (two doublings).
- A $1,000,000 nest egg left in plain cash has the buying power of about $492,000 in 24 years at 3% inflation. The Rule of 72 rounds that to "about half," and it is right.
This is the quiet danger of holding too much cash for too long: nobody takes the money from you, but it steadily buys less. To see the exact erosion for any amount and time frame, use the Inflation Calculator.
Why this matters more than the headline number
Inflation does not feel urgent year to year, but doubling math makes the long run brutal. A 3% annual increase sounds tame. Framed as "prices double roughly every 24 years," it is suddenly a retirement-planning problem. Someone retiring at 60 who lives to 84 will likely watch prices double once during retirement. Plan for one number and you may be short by half.
The flip side is the goal: your money has to grow faster than 72 / inflation to gain real ground. If inflation runs 3% and your savings earn 1%, your money is going backward in real terms even though the balance rises. Compare the two doubling times side by side:
| What is growing | Rate | Doubles in (Rule of 72) |
|---|---|---|
| Prices (inflation) | 3% | ~24 years |
| Cash in low-yield account | 1% | ~72 years |
| Diversified investments | 7% | ~10.3 years |
Money that doubles in 10 years is winning against prices that double in 24. Money that doubles in 72 years is losing. You can pressure-test your own return with the Investment Calculator or estimate doubling time for any return with the Rule of 72 calculator.
Using it to sanity-check raises and savings goals
A raise only keeps pace with inflation if your pay also doubles on the 72-divided-by-inflation schedule. At 3% inflation, a $60,000 salary needs to reach about $122,000 in 24 years just to stand still in real terms. If your raises average 3% a year, you are exactly treading water; below that, you are quietly getting a pay cut. Check the real value of a raise against inflation with the Pay Raise Calculator.
The same logic protects long-term savings targets. A "$1,000,000 retirement" set today is worth far less when you actually retire, so build the goal in future dollars. Tools like the Retirement Calculator let you inflate the target so you are aiming at the right number, not yesterday's number.
Where the shortcut breaks down
The Rule of 72 assumes one steady inflation rate, but real inflation jumps around year to year. Use it for a planning baseline, not a promise. For a single quick decision ("is my savings rate even keeping up?") it is excellent. For precise figures over a specific period, run the exact numbers in the Inflation Calculator instead.
For the official record of historical U.S. inflation, the U.S. Bureau of Labor Statistics Consumer Price Index is the authoritative, ad-free source.
The bottom line
Divide 72 by your inflation assumption and you have a fast, surprisingly accurate read on when prices double and your cash halves. Use it to spot when your savings are losing ground, then aim your investments and raises to beat that clock. Start with the Rule of 72 calculator and confirm the exact dollars with the Inflation Calculator.
Try it yourself
Run your own numbers in the free Rule of 72 Calculator — instant, private, no sign-up.
Open the Rule of 72 Calculator →Frequently asked questions
- How long until prices double at 3% inflation?
- Prices double in about 24 years at 3% inflation, found by dividing 72 by 3. The exact figure is 23.4 years, so the shortcut is within about half a year.
- What is the formula for prices doubling from inflation?
- Divide 72 by the annual inflation rate to get the approximate number of years for prices to double. At 4% inflation that is 72 / 4 = 18 years; the exact answer is 17.7 years.
- Does the Rule of 72 work for inflation as well as investing?
- Yes. Inflation is steady percentage growth just like investment returns, so the same shortcut applies. Dividing 72 by the inflation rate estimates when prices double, while dividing 72 by your return estimates when your money doubles.
- If prices double, how much buying power does cash lose?
- Cash loses half its buying power when prices double. At 3% inflation that happens in about 24 years, so $100,000 in cash would buy roughly what $50,000 buys today.
- What return do I need to beat inflation?
- Your money must double faster than prices do, meaning a higher rate than the inflation figure. At 3% inflation, prices double in about 24 years, so a 7% return that doubles money in about 10 years comfortably stays ahead in real terms.
- Is the Rule of 72 accurate for inflation?
- It is very close for normal inflation between roughly 2% and 6%, staying within a fraction of a year of the exact answer. It is a planning estimate, not a precise forecast, so use the Inflation Calculator when you need exact dollar amounts.
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