Ad request revenue per thousand impressions (RPM) is calculated by dividing your estimated earnings by the number of ad requests 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, 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. Learn more about the custom metric family.