What most calculators use: annual averages
This site, like most online inflation calculators, divides one year's average CPI-U by another's. The annual average is the number behind the official yearly inflation rate, it smooths out seasonal swings, and it gives every year exactly one value, which is what you want when the question is "what was 2000 money worth" rather than "what was money from a specific week of 2000 worth". The methodology page walks through the arithmetic, and the 2000-to-2025 pair page shows this exact conversion with its full context.
What the BLS calculator does differently: single months
The BLS's own CPI Inflation Calculator asks for a month, not a year, and divides one month's index by another's. Same series, different sampling, and the month you pick moves the answer: converting 2000 to 2025 month against month gives $186.17 using November and $188.19 using January, a $2.02 range inside a single index. Neither is more correct; a month-to-month conversion is the right tool when your dollar amount has a date on it, like a contract or a paycheck, and the annual average is the right tool for a year.
CPI-W: the Social Security number
The CPI-W tracks the same goods and services as the CPI-U but weights them for households whose income comes mostly from hourly-wage or clerical jobs, a subset of the urban population the CPI-U covers. It is the older population concept, with its own series back to 1913, and by law it drives the annual Social Security cost-of-living adjustment. Year to year it sits close to the CPI-U; over 2000–2025 the two cumulative rates differ by 0.3%. The gap grows with the horizon, as the table further down shows.
Chained CPI: the tax bracket number
The chained CPI-U, published since December 1999, updates its basket weights continuously to capture substitution: when beef gets expensive and shoppers buy more chicken, the chained index follows the switch within the month, while the standard CPI-U holds its basket fixed between periodic weight updates. The result is a measure that rises more slowly, and the gap is not small. From 2000 to 2025 the chained CPI-U rose 75.2% against the CPI-U's 87.0%, a 11.8% point difference that turns the same $100 into $175.19 instead of $186.96. Since 2018, federal income tax brackets have been indexed to the chained CPI-U, which is why brackets now creep up more slowly than the headline inflation rate. Its recent values are interim: the BLS revises each month's chained figure quarterly for about a year before finalizing it.
What about PCE?
The PCE price index is the fourth measure you will see cited, and the one this study's table leaves out because it comes from the Bureau of Economic Analysis, not the BLS. It is chain-weighted like the chained CPI-U, but it also covers spending the CPI never sees, such as health care paid by employers and government programs, and it draws its weights from business surveys rather than household ones. The Federal Reserve's 2% inflation target refers to PCE, not CPI. The guide to CPI vs. PCE covers the differences in detail.
Which number should you use?
Match the measure to the question. For what money from one year buys in another, use CPI-U annual averages, which is what this site's calculator computes. For an amount tied to a specific date, use the BLS calculator's month-to-month conversion. For Social Security benefits, the answer is CPI-W by statute, and for federal tax brackets it is the chained CPI-U, also by statute. For Federal Reserve policy, read PCE. A calculator is only "wrong" when it answers a different question than the one you asked.