Cost per mille

Cost per mille (CPM), where "mille" is Latin for one thousand, measures the cost of delivering one thousand advertising impressions. It is the oldest and most widely used pricing model in advertising, originating in traditional media (newspapers, television, radio) and carrying directly into digital advertising.

CPM is the default metric for campaigns where the goal is awareness and reach rather than direct response. Display advertising, video pre-rolls, social media brand campaigns, connected TV, digital out-of-home, and programmatic campaigns are all typically priced and evaluated on a CPM basis.

The metric matters because it answers a fundamental question about advertising efficiency: how much does it cost to put your message in front of people? A lower CPM means more eyeballs per pound of spend. But like all efficiency metrics, CPM must be evaluated in context. A cheap CPM on low-quality inventory, where ads appear below the fold, in irrelevant contexts, or are served to bot traffic, is not actually efficient. It is waste disguised as reach.

CPM is also used to compare the cost-efficiency of different media channels. You can calculate the CPM of a TV spot, a podcast sponsorship, a billboard, and a programmatic display campaign to compare how much each charges for a thousand impressions. This cross-channel comparison is imperfect because impression quality varies enormously, but it provides a useful starting point for media planning.

CPM is calculated by dividing total ad spend by total impressions, then multiplying by one thousand. If a campaign costs five hundred pounds and delivers two hundred and fifty thousand impressions, the CPM is two pounds.

The calculation is straightforward, but the definition of an impression varies by platform and measurement standard. Served impressions count every time the ad is sent to a browser, regardless of whether it is visible. Viewable impressions, as defined by the Media Rating Council, require that at least 50% of the ad's pixels are visible in the viewport for at least one second (two seconds for video). Viewable CPM (vCPM) uses viewable impressions as the denominator and is a more accurate measure of actual exposure.

Some platforms also distinguish between impressions and reach. Impressions count every time the ad is displayed, including multiple views by the same person. Reach counts the number of unique individuals who saw the ad. For brand awareness campaigns, reach-based metrics (cost per reach or cost per unique impression) can be more meaningful than raw CPM because they measure how many distinct people you have reached rather than how many times you have shown ads.

CPM sits at the top of the paid media metric tree, determining how much reach your budget purchases. From there, CTR determines how many of those impressions turn into clicks, and conversion rate determines how many clicks turn into outcomes. CPM is the entry point of the funnel and sets the ceiling on how many people you can reach.

The tree reveals an important relationship: CPM and CTR together determine CPC. If your CPM is ten pounds and your CTR is 1%, your effective CPC is one pound. Improving CTR while holding CPM constant reduces CPC, which reduces CPA. This means that creative quality (which drives CTR) and media buying efficiency (which drives CPM) are both levers for reducing acquisition cost, and the metric tree makes their relative impact visible.

1. 1

   Choose the right platform for your objective

   Each platform has a different CPM floor based on its audience quality and ad format. Match your platform choice to your campaign objective. If brand reach is the goal and your audience is broad, lower-CPM channels may be appropriate. If you need to reach a specific professional audience, a higher CPM on LinkedIn may be more efficient per qualified impression.

2. 2

   Broaden targeting to reduce CPM

   Narrow audiences command higher CPMs because there is more competition for fewer impressions. If your campaign goal is awareness, consider broader targeting with creative that self-selects the right audience. Not every campaign needs hyper-targeting.

3. 3

   Avoid peak competition periods

   CPMs spike during periods of high advertiser demand: Q4 for e-commerce, end-of-quarter for B2B, and major events. If possible, shift brand awareness campaigns to lower-competition periods to stretch your budget further.

4. 4

   Use frequency capping to avoid waste

   Showing the same ad to the same person twenty times does not generate twenty times the awareness. Set frequency caps to limit the number of times each individual sees your ad and redirect the saved impressions to new audiences.

5. 5

   Prioritise viewable and attention-based metrics

   Negotiate for viewable CPM (vCPM) rather than standard CPM. Use attention metrics where available to ensure your impressions are actually seen. A higher CPM with 90% viewability is better than a lower CPM with 40% viewability.