The Consumer Price Index measures and reports the effect of inflation and deflation on the economy. The Bureau of Labor Statistics calculates the CPI and publishes percentage changes. The Fed uses the CPI to determine whether to modify economic policies to prevent inflation. The BLS publishes the percentage change since the last month or last year.
https://www.thebalancemoney.com/consumer-price-index-cpi-index-definition-and-calculation-3305735

