What it means
An impression is one appearance of an advert on a screen, whether or not anybody reacts to it. Cost per impression divides total advertising spend by the number of impressions delivered, which tells a buyer the unit price of attention rather than the unit price of a result.
The metric matters because it is the working currency of brand advertising. When the goal is to be remembered rather than clicked, there is no sale to divide the spend by, so buyers compare channels on what it costs to put a message in front of a thousand pairs of eyes.
In practice, media buyers quote CPM rather than the per impression figure because it is far easier to read and negotiate. A $6.00 CPM means $6.00 buys a thousand impressions, or $0.006 each, and the same campaign priced at a $9.00 CPM is 50% more expensive for identical exposure.
Rates vary enormously with audience and format. General display inventory can trade in low single dollar CPMs, while tightly targeted business audiences or connected television slots run many times higher, which is why a low CPM on its own never proves a channel is good value.
The nuance that catches people out is that impressions are not people, and served is not the same as seen. Viewability standards exist precisely because an advert can be counted as delivered while sitting below the fold, so careful buyers compare viewable CPM instead of the raw figure.
Frequency is the other half of the picture, because the same budget can reach many people once or a few people many times over. Planners therefore read cost per impression alongside reach and frequency, since a thousand impressions spread across a thousand different people is a very different purchase from a thousand impressions shown fifty times to twenty people.
In practice
Real-world examples.
Example
A drinks brand compares two publishers quoting $4.00 and $7.00 CPMs. After checking viewability rates of 45% and 85% respectively, the finance lead works out the cheaper option costs $8.89 per thousand viewable impressions against $8.24, and the apparently expensive publisher wins the budget.
Example
A charity buys a national podcast sponsorship at a $22.00 CPM, far above its usual display buying. It accepts the premium because the audience is closely matched to its donor base and the host reads the message aloud.
Example
A retailer running an always-on awareness campaign watches its CPM creep from $5.20 to $8.90 across the fourth quarter. The media team explains that seasonal competition for the same audience bids up prices every year, and moves part of the budget into January.
Think of it
“CPM is what you pay for eyeballs on your ad-cost per thousand views.
Formula
Calculation
Cost per impression = total ad spend / total impressions delivered
CPM = (total ad spend / total impressions delivered) x 1,000
A consumer brand spends $24,000 on a display campaign that delivers 4,000,000 impressions. Cost per impression = $24,000 / 4,000,000 = $0.006, and the CPM is $0.006 x 1,000 = $6.00 per thousand impressions.
Suppose the platform reports that only 70% of those impressions met the viewability standard. Viewable impressions are 4,000,000 x 0.70 = 2,800,000, so the viewable CPM is ($24,000 / 2,800,000) x 1,000 = $8.57. The real price of attention is therefore about 43% higher than the headline rate suggested.Case study
Seen in the real world.
This is an illustrative, fictional scenario. Belmont Kitchenware, an invented homeware brand, spent $600,000 a year on display advertising and judged every campaign on the CPM it paid, always choosing the cheapest inventory available. Its average CPM of $3.20 looked excellent next to the $7.00 its competitors reported paying.
Brand tracking told a different story: prompted awareness had not moved in three years. A review found that the cheap inventory sat low on cluttered pages, with a viewability rate of only 38%, so Belmont's genuine viewable CPM was $8.42, worse than the competitor rate it had been quietly celebrating.
The fictional marketing director rewrote the buying rules to price everything on viewable CPM and accepted a headline CPM of $6.50 on better placements. Impressions delivered fell by roughly a third, viewable impressions rose, and awareness moved for the first time since the campaign began.
Watch out
Common mistakes.
- Treating impressions as individual people, when one person scrolling past the same advert five times generates five impressions.
- Choosing the lowest CPM available without asking about viewability, placement quality or audience match.
- Confusing CPM with cost per click, then wondering why a campaign generated millions of impressions and very little traffic.
Questions
People also ask.
What does the M in CPM stand for?
It comes from mille, the Latin word for thousand, so CPM is simply the cost per thousand impressions.
When is buying on impressions better than buying on clicks?
When the objective is awareness or recall rather than an immediate response, since exposure is the outcome you are actually paying for.
Is a $2.00 CPM always cheaper than a $10.00 CPM?
Not in any meaningful sense, because the value depends on who saw the advert and whether it was genuinely viewable.
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