What is an ad unit (ad slot) on a website?
What an ad unit is, how it is set up in an ad server, and what makes some ad units earn more than others.
Written by Samhub with the help of AI·Updated 13 Oct 2026·3 min readAn ad unit (ad slot) is a defined space on a page or in an app where an ad can appear, set up in the ad server with its sizes, position and targeting.
What defines an ad unit
| Setting | Example |
|---|---|
| Sizes | 300×250 and 320×320 on mobile |
| Position | Top of page, in article, sidebar |
| Page type | Front page, article, section |
| Key values | Section, topic or audience for targeting |
What makes an ad unit earn more
Viewability
Units readers actually see sell for more.
Size
Larger formats usually get higher prices.
Audience
Units with known readers attract data deals.
16 pages for publishers, with a checklist for data, programmatic and direct sales.
Fewer, better units
Many small, low-viewability units can lower overall prices and annoy readers. A simple setup with fewer, visible units often earns more per page.
CPM uplift at Stampen Media after activating first-party audiences with Samhub.
Frequently asked questions
They are often used the same way. Some use placement for a group of ad units sold together.
Enough to sell, not so many that viewability and reading suffer.
See our page on standard banner sizes.
Yes, multi-size units accept several sizes, which raises competition.
Related terms
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