how is inflation rate determined?

Statisticians collect price information on a variety of goods and services. They compare prices today with prices at some point in the past. That gives the inflation rate over that period of time.

They can fairly quickly calculate current rates based on key prices and indicators. Gas prices, car prices, house prices, food prices and so on are all either well reported or easily found.

Other kinds of statistics are harder to calculate on a current basis because it is harder to collect the basic information. That's why it is easy to keep current on inflation, but harder to keep current on something like gross domestic product. The basic information to calculate GDP takes longer to collect.

inflation rate cannot be determined but is rather forecast; inflation rate caused by market forces; usually the government of any country list out number of essential goods and services consume by that country and use it to compare with actual figure shown every month; when government changes goods and services basket then the actual figures will be shown after few months to compare month on month basis