Cost-Of-Living Adjustments for Federal Civil Service Annuities [Updated October 13, 2021] [open pdf - 995KB]
From the Summary: "Cost-of-living adjustments (COLAs) for the Civil Service Retirement System (CSRS) and the Federal Employees Retirement System (FERS) are based on the rate of inflation as measured by the Consumer Price Index for Urban Wage Earners and Clerical Workers (CPI-W). COLAs for both CSRS and FERS are determined by the average monthly CPI-W during the third quarter (July to September) of the current calendar year and the third quarter of the base year, which is the last previous year in which a COLA was applied. The 'effective date' for COLAs is December, but they first appear in the benefits issued during the following January. All CSRS retirees and survivors receive COLAs. Under FERS, however, nondisabled retirees under the age of 62 do not receive COLAs. Survivors and disabled retirees are eligible for COLAs under FERS regardless of age. CSRS pays a COLA that is equal to the percentage change in the CPI-W during the measurement period, but COLAs under FERS are limited if the rate of inflation is greater than 2.0%. [...] Congress passed the first law requiring automatic COLAs for federal civil service retirement benefits in 1962, and it has adjusted either the formula by which they are calculated or the date on which they take effect more than 10 times since then."
CRS Report for Congress, 94-834
Congressional Research Service: https://crsreports.congress.gov/