Ad request revenue per thousand impressions (RPM) is calculated by dividing your estimated earnings by the number of ad requests that you made, then multiplying by 1000.
Ad request RPM = (Estimated earnings / Number of ad requests) * 1000
For example, if you earned an estimated £60 from 15,000 ad requests, then your ad request RPM would equal (£60/15,000) * 1000, or £4.00.
Note that this metric is only available under the "Custom" metric family tab. Find out more about the custom metric family.