Cost per thousand impressions, usually called CPM, describes the cost of a thousand recorded advertising impressions. “Mille” is the origin of the M in the acronym.
It is a delivery-cost measure. It does not by itself describe unique people, attention, clicks or business outcomes.
How do you calculate CPM?
CPM equals advertising spend divided by impressions, multiplied by 1,000. If £500 buys 100,000 impressions, the CPM is £5.
Use consistent cost and impression definitions. A comparison becomes less useful when one figure includes different fees or a different delivery standard.
Impressions versus reach
Impressions count recorded displays under the platform's rules. Reach concerns the number of people or accounts reached according to its method.
The same person can generate several impressions. A low CPM therefore does not establish that the business reached a large number of different suitable people.
What is viewable CPM?
Viewable CPM uses impressions meeting a defined viewability standard. It is different from assuming every served impression was seen.
Check the platform's current definitions. Viewability is still not proof that a person understood or remembered the message.
What is a good CPM?
It depends on the audience, placement, format and objective. Cheap delivery to unsuitable audiences can have little value.
Compare relevant contexts and consider what the activity produces. A more expensive specialist audience may be useful if it fits the business and message.
How does CPM relate to clicks?
CPM concerns exposure cost, while click-through rate concerns recorded click response. Together they can help explain traffic economics.
Neither replaces outcome measurement. A campaign can generate inexpensive clicks without suitable enquiries or sales.
What should a report include?
Show spend, impressions and the relevant delivery context alongside CPM. Add reach, frequency and appropriate downstream evidence where available.
Explain the role of the campaign. An awareness activity and a direct-response campaign should not be evaluated through one unexplained ratio.
What should be avoided?
Do not optimise solely for the lowest visible unit cost. The resulting audience or inventory may differ from what the business actually needs.
A useful buying decision connects delivery conditions with a credible communication objective and an appropriate evaluation plan.
Further reading
For reference, see the supporting documentation. Explore related explanations of Pay-per-click advertising, Click-through rate, Impression share.
A practical example
Illustrative example: £300 of spend produces 60,000 recorded impressions, giving a £5 CPM. The team then reviews audience relevance, repeated exposure and the appropriate campaign outcomes, rather than treating the delivery price as proof of success.
Why it matters
CPM makes exposure cost comparable within a defined context. Clear impression and audience definitions prevent inexpensive delivery from being confused with useful impact.
