'Mille' is Latin for thousand, so CPM measures what it costs to show an ad 1,000 times. It's how Meta, YouTube, display networks, and connected-TV platforms price inventory, and it's the clearest gauge of how expensive it is to reach an audience in a given market, season, or placement.
CPMs rise with competition — the fourth quarter, election years, and narrowly defined audiences all push prices up — and they fall when ads are engaging enough that the platform rewards them with cheaper delivery. A sudden CPM spike with no change in targeting often signals creative fatigue or a crowded auction.
Example
A Meta campaign that spends $500 to generate 50,000 impressions has a $10 CPM ($500 ÷ 50,000 × 1,000).
How Sloss Digital uses it
We watch CPM as a market and creative signal — rotating fresh creative and broadening audiences when costs climb — while optimizing to the downstream metrics that pay the bills: cost per lead and return on ad spend.