CPM (Cost Per Mille) is the cost of showing an ad one thousand times. 'Mille' is Latin for thousand. The headline reach-efficiency metric for display, video, social and programmatic; it tells you what you pay for visibility before any click or conversion.
formula
$50K spent for 2.5M impressions = $20 CPM. Each thousand views cost twenty bucks.
why CPM matters
It doesn't directly produce sales, but it drives three big decisions:
- Channel selection · same audience, TikTok at $8 CPM vs Meta Reels at $15 CPM means TikTok wins for pure reach.
- Auction-pressure signal · rapidly rising CPM means competition spike; revisit bidding or targeting.
- Creative fatigue · flat CPM with rising frequency and falling CTR means creative refresh time.
CPM vs CPC vs CPA
- CPM: reach + visibility (TOFU).
- CPC: interest (MOFU).
- CPA: action (BOFU).
Different goals, all three needed. Just chasing low CPM buys empty reach; just chasing low CPA kills growth.
vCPM
YouTube and display count an impression as "viewable" if 50% of the ad shows for at least 1 second. vCPM (viewable CPM) is computed against viewable impressions only; far more meaningful than raw CPM. Use vCPM as the reporting metric for upper-funnel campaigns.