Effective cost-per-thousand impressions (eCPM) is a useful way to compare revenue across different channels and advertising programs. Essentially, effective CPM represents your estimated earnings for every 1000 impressions you receive.
Effective CPM doesn't represent how much you have actually earned -- rather, it's calculated by dividing your earnings by the number of page impressions, then multiplying by 1000. For example, if you earned $0.15 from 25 page impressions, then your eCPM would equal ($0.15/25)*1000, or $6.00. If you earned $180 from 45,000 impressions, your effective CPM would equal ($180/45,000)*1000, or $4.00.