Cost per Mille (CPM)

Technical explanation

CPM is calculated as advertising cost divided by measured impressions, multiplied by 1,000. It is both a buying model and a normalised delivery metric; the underlying impression definition and counting method must be consistent when comparing publishers, formats or campaigns.

Business relevance

CPM helps media teams compare the price of audience exposure, plan reach-oriented budgets and monitor auction or inventory costs independently of clicks and conversions.

Implementation example

A campaign spends £4,800 and records 1,200,000 impressions. Its CPM is (£4,800 ÷ 1,200,000) × 1,000 = £4.

Limitations and common misconceptions

CPM measures delivery cost, not attention, unique reach, viewability, engagement or business outcomes. Invalid traffic, repeated exposure and inconsistent impression standards can distort comparisons.

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