CPM means cost per mille, where mille is Latin for thousand, so it is the price of one thousand impressions. An impression is counted each time an ad is served to a screen, whether or not the person notices it, scrolls past it or clicks.
The metric shows up wherever inventory is sold by exposure: display networks, video placements, connected TV, podcast sponsorships and paid social. Meta reports CPM on every campaign regardless of the objective, and programmatic platforms price nearly all of their inventory this way.
Because CPM says nothing about what happened after the impression, it belongs at the top of the funnel. A retailer paying to introduce a new skincare line reads CPM; the same retailer paying for purchases reads CPA or return on ad spend and treats CPM as a supporting number.
Mechanics
What pushes CPM up or down
Ad platforms sell impressions through an auction as well, but the winning bid is measured per thousand views. The clearing price depends on how many advertisers want the same audience at the same time, which is why CPM climbs during the holiday season and in the weeks before an election.
The audience you pick matters as much as the calendar. A narrow group of people who recently visited your pricing page is expensive per thousand because many advertisers chase the same visitors. A broad audience of adults in one state is far less contested, so each thousand impressions costs less, even though fewer of them will care.
Format and placement change the number too: a vertical story video, a skippable pre-roll and a small banner under a news article are sold at very different rates. Frequency plays a part as well, since the tenth exposure to the same person still counts as an impression and still costs money.
Example
A worked example for an online store
Suppose an online store selling kitchenware launches a video campaign on Meta to introduce a new cast iron line. Over two weeks it buys 800,000 impressions among people who follow cooking pages, for a total spend of 4,800. CPM is 4,800 divided by 800, which comes to 6.00 per thousand impressions.
To see whether that reach was worth paying for, connect it to the next step. Say the video earns a 1.5 percent click-through rate, so 12,000 clicks, and 2 percent of those visitors buy, so 240 orders. At an average order of 60, the campaign produced revenue of 14,400 against spend of 4,800, and each order cost 20 to acquire.
Change one assumption and the picture shifts. With a click-through rate of 0.5 percent the same 800,000 impressions deliver 4,000 clicks and 80 orders, and the cost per order rises to 60, which equals the average order size.
Use
When to lean on CPM and when to look past it
- Lean on CPM when the goal is to be seen by a defined group: a product launch, a brand campaign, an event announcement or a message that has to land before a decision is made.
- Use it to compare placements that sell the same audience, such as two video networks or two segments inside one Meta account, where the only variable is the price of exposure.
- It misleads when a low CPM comes out of a broad or low quality audience; you bought a lot of impressions, but the wrong people saw them.
- It misleads for sales campaigns, where the platform optimizes toward purchases and a higher CPM is often the sign that it found buyers rather than scrollers.
- Pair it with CTR and frequency to learn whether people reacted, and with engagement rate when the format invites comments and shares.
Watch out
Common mistakes with CPM
- Choosing the lowest priced impressions available and then blaming the creative when the campaign produces no lift in searches, visits or sales.
- Reading CPM as a measure of results, when it only prices the opportunity to be noticed and says nothing about what people did afterward.
- Comparing CPM across formats, such as a six second bumper against a full screen story, as if a view in each one were worth the same.
- Letting frequency run unchecked, so a large share of the impressions you pay for are the eighth or ninth exposure to the same person.
- Forgetting viewability: an impression served below the fold on a page nobody scrolls still counts, and still costs, on many display networks.