Average CPM vs impression-weighted CPM
Combined CPM is total money divided by total impressions, multiplied by 1,000. Segments with 1,000 impressions at 10 CPM and 99,000 impressions at 1 CPM have a combined CPM of 1.09, not the simple average of 5.50.
Average CPM vs impression-weighted CPM
| Segment | Impressions | Segment CPM | Money at that rate |
|---|---|---|---|
| A | 1,000 | 10.00 | 10.00 |
| B | 99,000 | 1.00 | 99.00 |
| Combined | 100,000 | 1.09 | 109.00 |
| Unweighted mean of the two CPMs | Not applicable | 5.50 | Not a combined revenue estimate |
Formula & method
Combined CPM = 1,000 × Σ money ÷ Σ impressions
Equivalent weighted form = Σ(CPMi × impressionsi) ÷ Σ impressionsiUse the underlying totals whenever possible. The same calculation works for advertiser cost CPM and publisher revenue eCPM, but do not mix cost with revenue. If all segments have the same impression count, the arithmetic and weighted averages coincide; otherwise the larger segments should carry more weight.
Keep currencies, time zones and impression definitions aligned. Rounded CPM values can introduce reconstruction error, so a platform’s exact money and impression totals are better inputs than its displayed two-decimal rates.
Worked example
At 20,000 impressions and 2.50 CPM, one segment contributes 50.00. Another 80,000 impressions at 0.75 contributes 60.00. Together, 110 ÷ 100,000 × 1,000 gives 1.10 CPM.
What can change the answer
A combined CPM does not describe the typical individual visitor or forecast future rates. A segment with zero impressions has no defined CPM.
Sources & definitions
Method checked:
Continue with a related task
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CPA equals advertising spend divided by tracked actions. A campaign costing 600 with 24 qualifying actions has an average CPA of 25.