TIL that to calculate the CPI for a single item, you would simply do CPI = (updated cost / base period cost) x 100. The "updated cost" is divided by that of the initial year, then multiplied by 100.

https://en.wikipedia.org/wiki/Consumer_price_index

https://www.reddit.com/r/todayilearned/comments/15ndyfc/til_that_to_calculate_the_cpi_for_a_single_item/