CPM, impressions and reach: estimate an advertising budget
Keep displayed impressions separate from the people who may have seen them.
Method
CPM is spend ÷ impressions × 1,000. Rearranging gives impressions = spend ÷ CPM × 1,000. Reach counts distinct people or devices according to the platform’s definition, whereas one person can produce several impressions. Approximate average frequency = impressions ÷ reach when both totals use matching measurement rules.
Worked example
With a budget of 600 and CPM of 8, the estimate is 600 ÷ 8 × 1,000 = 75,000 impressions. If reported reach is 25,000, average frequency is 3. That does not mean every person saw exactly three impressions. At a measured 1% click-through rate, 75,000 impressions imply 750 clicks as an arithmetic scenario.
What to check
CPM can change with auction conditions, inventory and billing definition. Estimated clicks are not guaranteed visits or sales. Do not infer reach from impressions alone without a frequency assumption, and do not compare viewable impressions with served impressions as though they were the same measurement.