The two definitions, from their owners
Google Ads defines CPM for the buyer: the amount an advertiser pays per 1,000 impressions of an ad Google Ads Help2026-08. AdSense defines Page RPM for the seller: estimated earnings per 1,000 pageviews Google AdSense Help2026-08. Three structural differences follow:
| CPM (buyer) | RPM (seller) | |
|---|---|---|
| Who sees it | Advertiser | Publisher |
| Counts per 1,000 | Impressions of one ad | Pageviews, sessions or views |
| Platform share | Not yet deducted | Already deducted |
| Scope | One campaign or unit | All ads on the property |
Worked forward: from a CPM buy to your RPM
advertiser pays $2.00 CPM for one ad unit
platform keeps its share; you net $1.50 per 1,000 impressions on that unit
your page runs 4 units, 80% average fill = 3.2 impressions per pageview
page RPM = $1.50 × 3.2 = $4.80
Note what did the work: ads per page and fill multiplied the per-unit number; the revenue share shrank it. The 25% cut in this example is illustrative arithmetic; real shares are on each network's page in the network table, and only the published ones appear there.
Worked backward: from your RPM toward the buy side
your page RPM = $6.00 with 5 units at 75% fill = 3.75 impressions/pageview
net per 1,000 impressions = 6.00 ÷ 3.75 = $1.60 impression RPM
gross CPM upstream = higher than $1.60 by the platform's share
You cannot recover the exact advertiser CPM without knowing the share applied to each impression, which is why RPM-to-CPM converters that promise one clean number are quietly making one up.
Why the confusion persists
YouTube shows creators both numbers side by side, which helps and confuses in equal measure: CPM there covers only monetized playbacks while RPM divides by all views. That special case gets its own page: YouTube RPM vs CPM. And if the buyer-side metric is what you actually need, the sister reference whatiscpm.com treats CPM at full depth.