Index base period
The period set equal to 100 in a price index, for example 2015 = 100.
2 min read · Reviewed September 2026
In plain words
Every price index needs a reference point. Setting 2015 = 100 means an index of 132 in 2024 tells you prices are 32% above 2015. The base period itself is arbitrary — it doesn't affect the inflation rates you calculate from the index.
Frequently asked questions
What does 2015 = 100 mean on a price index?
It means the index was set to 100 in 2015. An index reading of 132 in 2024 means prices are 32% above their 2015 level. The base year is arbitrary — changing it doesn't affect any inflation rate you calculate.
Further reading