How to calculate eCPM
Divide total ad revenue by the number of impressions and multiply by 1,000. A service that earned $4,200 from 600,000 impressions has an eCPM of $7.
A stricter version divides by ad opportunities, every slot that could have held an ad, instead of by ads actually shown. This is sometimes called revenue per mille (RPM) and is the truer measure of what your inventory earns.
Why eCPM differs from CPM
For example, half your slots sold at a $12 CPM and half unfilled gives $6 per thousand opportunities.
- Fill: unsold slots earn nothing and pull the average down
- Mixed sources: direct deals at high rates and open auctions at low ones are blended
- Other pricing models: ads paid per click or per completed view are converted to a per-thousand equivalent
- Intermediaries: networks and exchanges take their share before you are paid
Using eCPM
Because it folds price and fill together, eCPM is the figure to use when comparing ad partners, devices, regions or content types. A partner with a lower quoted CPM and near-complete fill can earn you more than one with a high CPM that fills a fraction of requests.
Use it in revenue planning too: multiply expected ad opportunities by eCPM, not by the best CPM you have been offered.
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