The Consumer Price Index rose 3.7% over the year ending in September. That's a useful picture of how prices are changing across the economy, but it doesn't mean the cost of your own life rose by exactly 3.7%.
Maybe you have a fixed mortgage payment while your health insurance premium keeps climbing. Maybe you rent, drive a lot, or spend more on travel than the average household. Where you spend your money determines how much each price increase affects your budget.
Traditional FIRE planning already accounts for inflation. The question is how the prices you actually pay compare with the assumptions you're using to estimate your future spending. Your portfolio won't be paying for the average American's groceries or housing. It'll be paying for yours.
You don't need to build your own government statistics agency to account for that. You can make a reasonable estimate from your spending, then use it to examine the assumptions behind your FIRE number.
🛒 Your Household Buys a Different Basket
The CPI tracks price changes across a broad basket of goods and services. Some prices rise faster than others, so the overall rate depends partly on how much weight each category carries in the index.
Your household has its own weights. If you spend a lot on something whose price rises quickly, that increase has a bigger effect on your budget than it does for someone who barely buys it.
Let's say a household spent $80,000 last year. For the moment, assume they buy the same quantities of the same things this year, but the prices change as shown below. These are hypothetical expenses and price changes, not the government's category measurements.
Health costs went up 8%, but health costs weren't the entire budget. The fixed mortgage payment didn't rise, and the other categories increased by different amounts. Together, the same basket costs $2,750 more than it did last year.
Divide that increase by the original $80,000, and you get an estimated household inflation rate of 3.4%. Depending on the household and the year, the result could be higher or lower than CPI.
There's a catch when you try this with your own records: spending more doesn't always mean prices went up. If you took an extra vacation, bought a bigger car, or added a streaming service, some of the increase paid for more or different things. A simple comparison of total spending captures those choices along with price changes.
You don't have to separate every receipt perfectly. But if you're trying to estimate how much more the same life costs, it helps to distinguish a higher premium for the same health coverage from the decision to buy a different policy.
🏠 A Fixed Mortgage Changes Your Bills
Housing is a good example of why your cash needs and CPI can move differently.
For owner-occupied homes, the CPI uses owners' equivalent rent: an estimate of the price of the housing services an owner receives, based on rental prices. It doesn't use your mortgage payment as the price of living in your house.
If you have a fixed-rate mortgage, your principal and interest payment can stay the same even while rents rise. That stable payment affects how much cash you need each month, and it deserves a place in your retirement spending calculation.
Of course, the mortgage payment isn't the whole cost of owning a home. Property taxes, insurance, utilities, and repairs can still increase. And if you eventually pay off the mortgage, move, or buy another house, your future housing costs may look very different from the ones you have today.
The CPI's housing measure and your household budget are measuring different things. Both can be useful, as long as you know which question you're asking.
The free FIRE BTC Compass can show you how your current spending connects to your FIRE target. Add your expenses and investments to see your FIRE level and compare traditional and bitcoin-adjusted targets. Your financial information stays in your browser.
The next step is figuring out what spending amount to use—and how your target might change if those costs rise faster than you expect.
