Glossary
Base month
The earlier of the two months in a CPI ratio. Not the same thing as the index's reference base, such as 1982–84=100.
In a CPI clause, the base month is the month whose index level sits on the bottom of the ratio. A clause that adjusts September 2026 fees using the change from September 2025 has September 2025 as the base month and September 2026 as the comparison month.
This is easy to confuse with the index base period printed by BLS, such as 1982–84=100. That base period is how the index points are scaled. It is not the month in your contract. You do not need to rebase the index to 100 before dividing. Divide the published levels.
Some clauses always go back to the original month of the contract (see original-base method). Others step one year at a time and then compound. The base month is different in those two readings, and the dollars diverge over a long agreement.
Publication lag matters. The index for a month is released the following month. A clause that needs the September index cannot be computed in September. It can be computed in October, when BLS has published it. Build the lag into the notice calendar.
Related terms
See it used in a worked example · CPI calculator · Request an audit
Sources and limits
Index levels from the U.S. Bureau of Labor Statistics Consumer Price Index, not seasonally adjusted, retrieved through the BLS Public Data API on November 1, 2026. Values are index points, not the headline 12-month percent.
Statewide rates reflect U.S. Department of Labor state minimum-wage postings as updated January 1, 2026, the U.S. Department of State Diplomatic Note on 2026 increases, and, for the prior-year comparison and named local rates, the National Employment Law Project compilation published in November 2025. Confirm the rate that applies to the workplace before using it in a bill.
The contracts in these worksheets are fictional. The prices are chosen so the arithmetic is easy to check. They are not a price survey and they are not client results.
RateClause is not a law firm. Nothing on this page is legal advice. Your clause controls, including whether a past period can be rebilled at all.
Primary postings: BLS Consumer Price Index, U.S. Department of Labor, state minimum wages, U.S. Department of State notice of 2026 minimum-wage increases.