Metric wiki
CPM vs eCPM
By Oliver Wakefield-Smith · updated July 2026
The short answer
CPM is a contracted price; eCPM is a measured outcome: earnings divided by impressions, times 1,000, whatever the money's pricing model was. When a report shows both and they disagree, one of three causes is responsible: mixed pricing underneath, unfilled impressions in the denominator, or two systems counting impressions differently.
Cause 1: mixed pricing models
eCPM blends everything that earned
100,000 impressions: $300 of CPC revenue + $150 from a $3.00-CPM deal on half of them
eCPM = $450 ÷ 100,000 × 1,000 = $4.50, above the contracted $3.00 CPM
Illustrative blend; the point is the mechanism, not the values.
Cause 2: unfilled impressions
eCPM usually divides by ALL impressions, sold or not. A publisher filling 70% of inventory at a $10 contracted CPM measures a $7 eCPM across total traffic. Nothing went wrong; the metrics answer different questions (price of the sold thousand vs yield of every thousand). Fill-rate context lives with the supply chain at programmatic CPM.
Cause 3: discrepancy
Your ad server, the buyer's DSP and the network dashboard count impressions at different pipeline stages (requested, rendered, measured), so the same campaign produces different denominators in different reports. Single-digit-percent gaps are routine plumbing; larger gaps are a reconciliation project, not a metric mystery.
Which number to trust
- Buying decisions: the contracted CPM, since it is what you agreed to pay.
- Yield decisions: eCPM, since it is what the inventory actually returned; its full definition and worked conversion are at what is eCPM.
- Cross-chain reconciliation:neither alone; the CMA's ~35% intermediary finding is the reminder that buyer CPM and publisher eCPM are separated by a whole industry.
Seller-side sibling metric: RPM. Base arithmetic: the formula.