CPI stands for Consumer Price Index. It is the most popular reference for day-to-day inflation. CPI gets calculated as a measurement of price change using a weighted average basket of consumer goods and services purchased by households.
The annual percentage rate change in CPI gets used as a measure of inflation to decide wages, salaries, and pensions. Governments and central banks use the inflation rate to determine fiscal and monetary policy.
National statistics agencies such as the ONS in the UK and BLS in the United States, usually calculate the metric and publish their results monthly. Each month, both monthly and yearly CPI (inflation) figures get published by the statistics agencies.