You asked
The consumer price month index number for November 2017 was 104.6:
https://www.ons.gov.uk/economy/inflationandpriceindices/timeseries/d7bt/mm23
When compared to the November 2016 index number of 101.4 this is a 104.6/101.4 = 1.031558 = 3.16% increase over 12 months.
So why is the reported CPI 12-month percentage change figure for November 2017 3.1% rather than 3.2%?
We said
Thank you for your query.
The rates are calculated from unrounded index numbers (i.e. they are to more than 1 decimal place). Each month we publish the index numbers to 3 decimal places (see table 57 in this file https://www.ons.gov.uk/economy/inflationandpriceindices/datasets/consumerpriceinflation).
Using the unrounded indices: 104.557/101.421=3.09%