What a CPI contract settles on, and why a revision never changes it
Kalshi's CPI markets settle on the seasonally adjusted one-month change in headline CPI-U, rounded to one decimal, as the Bureau of Labor Statistics first publishes it. Core inflation is a separate series with its own markets, and a later revision by the BLS never re-settles a contract.
Headline, not core
The contract terms name the underlying exactly:
Underlying: The Underlying for this Contract is the signed one-month, one-decimal percent change in the seasonally adjusted Consumer Price Index for All Urban Consumers (CPI-U) published by the Bureau of Labor Statistics (“BLS”).
The September 2026 market reads:
If the Consumer Price Index (CPI) increases by more than 0.1% (single-decimal) in September 2026, then the market resolves to Yes.
Core CPI trades as its own series, KXCPICORE, with a rule of its own:
If the seasonally adjusted Consumer Price Index for All Urban Consumers: All Items less Food and Energy for December 2026, as published by the Bureau of Labor Statistics, increases by above 1.0%, then the market resolves to Yes.
Revisions never re-settle
The value that counts is the one published at expiry:
Revisions to the Underlying made after Expiration will not be accounted for in determining the Expiration Value.
The market also closes before the number lands:
The market will always close at 8:25 AM ET on the scheduled day of the data release (October 14, 2026). Please note: the Expiration Value is the single-decimal value published at the Source Agency. In the case of a delay in data caused by a federal government shutdown impacting the reliability of the Source Agency, the market’s latest Expiration Date will be extended to the sooner of the release of the Underlying or six months after the end of the government shutdown.
Polymarket's CPI markets measure something else
Polymarket's September markets include an annual headline market, which is not seasonally adjusted, and a monthly core market. Neither is the same question as Kalshi's monthly headline contract:
This is a market about inflation over the 12-month period ending September 2026, before seasonal adjustment, as reported by the Bureau of Labor Statistics. This market will resolve to the percentage change in the Consumer Price Index (CPI) over the 12-month period ending in September 2026 according to the monthly Bureau of Labor Statistics (BLS) report. The resolution source for this market will be the BLS Consumer Price Index report released for September 2026 (https://www.bls.gov/bls/news-release/cpi.htm), currently scheduled to be released on October 14, 2026, at 8:30 AM ET. Resolution of this market will take place upon release of the aforementioned data. Note: the resolution source for this market will be the official monthly BLS CPI news release, which reports inflation over 12-month periods to only one decimal point (e.g., 2.9%). Thus, this is the level of precision that will be used when resolving the market. If the BLS does not release the relevant figures on the scheduled date, this market may remain open up until the scheduled release time of the next CPI report (https://www.bls.gov/schedule). If the information is not released by that time, this market will resolve according to the figures of the most recent previous month with available data.