To calculate eCPM which is short for Effective Cost per 100 impressions or Blended CPM you take the total dollar amount spent for a given time period and you divide by the amount of impressions served then multiply answer by 1,000.
Example: Tori is managing four campaigns. It’s July 1st and she needs to figure out the eCPM for all of the four campaigns combined for the previous month (June). Her total media cost for June was $11,374 and the total amount of impressions served across the four campaigns is 4,973,094.
To Calculate eCPM or Blended CPM, she would take the $11,374 media cost, divide it by the 4,973,094 impressions served then multiply by 1,000 to get an eCPM or Blended CPM of $2.29. (11,374/4,973,094*1,000 = 2.287)