How can I calculate a non-calendar year annual average percentage change using the CPI?
In order to find an annual average percentage change, you must divide the current non-calendar year annual average index by the previous non-calendar year annual average index. To do this, you must first obtain 24 monthly indexes (12 indexes from the current non-calendar year and 12 indexes from the previous non-calendar year).
Once you have obtained the 24 monthly indexes, you must calculate the arithmetic average of the 12 monthly indexes for each non-calendar annual period. To find an annual average, you must add the 12 indexes for each annual period, then divide the number by 12.
Using the non-calendar year annual average indices that you have just calculated, follow the formula below:
(Current annual average index / previous annual average index – 1 ) x 100 = the annual average percentage change (%)
Finally, round the result to one decimal place.