The base period of the index is the period when the index has a value of 100. For example, an index base 100 in 2005 means that the average of the twelve indices for the year 2005 is 100. The index value for a given month is compared with its value in the base period. The national consumer price index is published on base 100 in 2005 and on base 1.1.1948 as well. The conversion of an index from one base to another is achieved by multiplying (or dividing) the index by a fixed factor.

Example: Conversion for August 2008 of base 100=2005 to base 100=1.1.1948.