eCPM (effective cost per mille) is what the ad platform thinks one impression of your ad is worth to it, scaled to a thousand impressions. It’s the auction’s ranking number: the lens through which the platform decides whether your bid is competitive against everyone else chasing the same impression.
A small example makes the arithmetic clear. A buyer is on target-CPA at $30. The platform predicts a 2% CTR and a 5% CVR for this impression. Expected revenue per impression is roughly $0.03 (2% × 5% × $30), and scaled to a thousand that’s a $30 eCPM. Those percentages are illustrative, not benchmarks.
The formula’s shape is (estimated bid × estimated CTR × estimated CVR) × 1,000, with platform-specific quality and floor adjustments layered on. The advertiser doesn’t set eCPM directly. You set a goal (target CPA, max CPC, value bid, target ROAS) and the platform derives the eCPM estimate from it. That estimate is what places you in the auction.
The operator lever here is creative. Because eCPM scales linearly with predicted CTR and CVR, a 30% CTR lift raises eCPM by roughly 30% at constant bid and CVR — the same rank move as a 30% bid increase, with no extra spend. It’s also why platform-wide CPMs drift up when aggregate eCPM rises: every advertiser’s rank moves together, so clearing prices climb. The “my CPC went up without me changing bids” pattern is usually this story.
eCPM is the platform’s pre-auction estimate, used to rank bidders; CPM is the post-auction price the winner pays. Publisher-side eCPM (what a publisher earns per thousand impressions across networks) is the same arithmetic from the other side of the impression.