Formula and its place among buying models
CPM = spend / impressions × 1000
Example: $1,800 / 3,000,000 × 1000 = $0.60 per thousand impressions
CPM is the buying unit in which the advertiser pays for the delivery of a message, not for a reaction to it. All the risk of inefficiency sits on the advertiser’s side: the impressions are paid for whether anyone clicked or not.
| Model | Billable unit | Who carries the risk | Typical job |
|---|---|---|---|
| CPM | A thousand impressions | The advertiser | Reach, product launch, demand support |
| CPC | A click | Split: the platform answers for attention | Traffic acquisition |
| CPA | A target action | Formally the platform | Direct sales, lead generation |
eCPM: one scale for different deals
A placement bought on clicks cannot be compared directly with a placement bought on impressions — the units differ. eCPM brings both deals onto one scale:
eCPM = spend / impressions × 1000
eCPM = CPC × CTR × 1000
The second identity is handy before launch. Example:
CPC = $0.60, CTR = 0.8%
eCPM = 0.60 × 0.008 × 1000 = $4.80
In other words, buying clicks at this click-through rate is equivalent to buying impressions at $4.80 per thousand. If the same audience is available at a CPM of $3.00, the reach buy works out cheaper per click — provided the CTR holds.
| Placement | Model | CPM / eCPM | Comment |
|---|---|---|---|
| Display banner | CPM $2.40 | $2.40 | Price known in advance |
| Search advertising | CPC $0.60, CTR 5% | $30.00 | Expensive per impression, but the impression is highly targeted |
| Shopping ad | CPC $0.24, CTR 1.2% | $2.88 | Comparable to display on the price of reach |
| Sponsored block in a catalogue | CPM $6.00 | $6.00 | A premium for a slot on the shopper’s path |
The conclusion from the table is not “display is cheaper” but “a single number means nothing without the quality of the impression”. A thousand impressions in search results on a transactional query and a thousand impressions in a banner network are incomparable events, brought to one scale.
CPM in retail media
Retail media is the retailer’s own advertising inventory: paid positions in the shop’s search results, in category listings, on the product page and in recommendation blocks. The buyer of that inventory is a supplier or a brand; the seller is the retailer.
CPM took hold here as the main pricing unit for three reasons:
- Inventory volume is predictable. The retailer knows the traffic of its categories in advance and can sell impressions as capacity.
- Conversion does not depend on the retailer alone. After the impression, the product page, the price, availability and reviews take over — all of which the supplier controls. Selling an action would be commercially unattractive and hard to justify.
- The audience is already in a buying state. An impression next to a product on the shop’s shelf is worth more than an impression in the open network, and that premium is priced straight into the CPM.
As a result, retail media pricing is almost never discussed apart from two parameters.
| Parameter | Why it is decisive |
|---|---|
| Placement slot | Top search positions, the top of a listing, a block on the product page — fundamentally different visibility at the same price per thousand |
| Viewability | An impression below the fold that nobody scrolled to is formally counted but delivered no contact |
| Query context | An impression on a category query and on a competitor’s brand query are events of different value |
| Frequency per user | Without frequency capping, reach quietly turns into repeats against one narrow audience |
| Reporting transparency | Served impressions, viewable impressions and unique reach are three different numbers in one report |
A cheap CPM almost always means a poor slot. Before comparing offers, convert them to a price per thousand viewable impressions and check whether the reach is being assembled from positions the user physically cannot see, or stopped noticing long ago because of banner blindness.
When CPM is the right choice
CPM makes sense if at least one condition holds:
- The job is presence, not an immediate click: launching a new product, supporting a seasonal peak, keeping the brand visible in a category with a long decision cycle.
- The format has a known high CTR. With good click-through, buying impressions delivers a lower effective CPC than buying clicks directly.
- You need control over reach and frequency. The impression model lets you manage how many people saw the message and how many times — that cannot be set on a click basis.
- The slot is scarce. Premium positions are sold on impressions simply because demand for them exceeds supply.
The opposite signals are a narrow transactional task, a short purchase cycle and a requirement to report on cost per order. In those cases it makes more sense to buy clicks and use CPM as a secondary measure for comparing placements with one another and for keeping the price of reach in line with revenue per thousand visitors.