How to Calculate Cpi?

Each month the Beauru of labor statistics calculates the CPI every month. They do this by taking the inflation rate and measuring the type of goods and the time frames sense people want to know the price increase over a certain time. Secondly, they calculate the number of units that have been purchased, rinse then repeat step two then take the amount every month one year ago by the amount present day and lastly take the amount in the third step and multiply by 100 and this is the rate increase.