RPM vs CPM for Publishers

CPM

CPM often describes what an advertiser pays or a pricing basis for impressions.

RPM

RPM describes publisher revenue normalized per thousand units such as pageviews or impressions.

Avoid mixing denominators

Page RPM and impression RPM are not interchangeable because one uses pageviews and the other uses ad impressions.

Practical takeaway

Use the metric that matches the business question you are trying to answer. When several metrics describe different stages of the same workflow, compare them together rather than optimizing one in isolation.