People quote them interchangeably and misprice everything downstream. They are not the same metric. Definitions, with one site's numbers, n=1 illustrative, 28d:
— CPM = revenue per 1000 impressions = $5.00
— Impressions/pageview = 4.2
— RPM = revenue per 1000 pageviews = CPM × imps/pv = $21.00
Reporting "my CPM is $5" while comparing to someone's "$21 RPM" is a 4x apples-to-oranges error. Always state which.
So what: CPM × impressions-per-page = page RPM. Label the unit or you'll misjudge 4x. ▲0% confusion if you label it.
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Myth: RPM and CPM are basically the same
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