The inflation rate that's mostly used in the US economy is calculated from

Every month the US Bureau of Labor Statistics gathers price information on specific items from thousands of stores, service establishments, rental units and doctors' offices, in order to measure price changes in the CPI (Consumer Price Index). The CPI measures price changes in consumer products and services which are representative of the economy such as food, gasoline, clothing, vehicles, etc. This CPI data appears at the Bureau of Labor Statistics. As a result, the inflation rate is calculated from the Consumer Price Index.