Cost per mille (CPM)
Cost per mille (CPM) is the cost of one thousand impressions — 'mille' is Latin for thousand. It is the standard pricing unit for awareness and display buying, where advertisers pay for exposure rather than clicks. CPM depends entirely on how an impression is defined (served vs viewable), and it says nothing about whether anyone clicked or converted, so it is an exposure-cost metric only.
What this means
CPM = (total cost ÷ impressions) × 1000. It is the price of reaching a thousand impressions. Because it is charged on exposure rather than response, CPM is common for brand-awareness and reach campaigns where the goal is to be seen, not necessarily clicked.
CPM and the impression definition
CPM is only as meaningful as the impression behind it. A 'served' impression may load off-screen and never be seen, while a 'viewable' impression must meet the IAB/MRC standard (at least 50% of pixels visible for at least one continuous second for display). A low CPM bought against served impressions can be more expensive in real, viewable terms than a higher CPM bought against viewable ones. CPM is distinct from eCPM, which back-calculates an effective cost-per-thousand from other pricing models.
- CPM = cost per thousand impressions
- Served vs viewable impressions change what CPM really buys
- Charged on exposure, not on clicks or conversions
How it appears in analytics and logs
A CPM figure tells you what a thousand impressions cost. Whether that exposure was worthwhile depends on viewability and downstream action, neither of which CPM captures.
Diagnostic use case
Use CPM to compare the cost of buying exposure within one platform's impression definition, and pair it with viewability and outcome metrics before reading it as value.
What WebmasterID can help detect
WebmasterID measures what visitors do after they arrive via first-party events, so CPM-bought exposure can be judged by real on-site outcomes rather than impression counts alone.
Common mistakes
- Comparing CPM across served and viewable impression definitions.
- Reading CPM as a response or value metric.
- Confusing CPM with eCPM (effective cost per mille).
Privacy and accuracy notes
CPM is a cost-per-thousand-impressions ratio; it is aggregate and needs no personal identifiers to compute.
Related pages
- Effective cost per mille (eCPM)
Effective cost per mille (eCPM) expresses earnings as revenue per thousand impressions, regardless of how the inventory was actually priced. It lets a publisher compare a CPC deal, a CPA deal, and a CPM deal on one common scale by back-calculating what each earned per thousand impressions. eCPM is a normalization metric — it measures yield, not the contract terms — and it depends on the same impression definition issues as CPM.
- Viewability rate
Viewability rate is the percentage of measured ad impressions that qualified as viewable under an industry standard, rather than merely served. The IAB and MRC define a viewable display impression as at least 50% of the ad's pixels in view for at least one continuous second (two seconds for video). The rate exposes the gap between ads delivered and ads actually given a chance to be seen.
- Cost per click (CPC)
Cost per click (CPC) is the amount an advertiser pays for each click, calculated as total cost divided by clicks. In an auction-based system the actual CPC is set by competing bids and ad quality, not just your max bid. CPC measures the price of a click, not its worth — a cheap click that never converts is not a bargain — so it is read alongside conversion and value metrics, never alone.
- Website observability
See what exposure-bought traffic does on-site.
Sources and verification notes
- IAB/MRC — Viewable ad impression measurement guidelines
- Google Ad Manager Help — CPM, CPC, CPA, and vCPM
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.