Ad revenue per thousand impressions (RPM) is calculated by dividing your estimated earnings by the number of ad impressions you received, then multiplying by 1000.
Ad RPM = (Estimated earnings / Ad impressions) * 1000
For example, if you earned an estimated £180 from 45,000 ad impressions, then your ad RPM would equal (£180/45,000) * 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.