Basics · also called inventory, ad space, supply
Ad inventory
Ad inventory is the total amount of advertising space a publisher has available to sell, usually counted in impressions across its ad slots, pages, apps or screens.
Inventory is the stock a publisher sells. On the web it is counted as potential impressions: every page view multiplied by the number of ad slots on it. In video, it is the ad breaks in each stream; in audio and podcasts, the spots in each episode; in DOOH, the plays on each screen.
Inventory is perishable. An impression that is not sold when the page loads is lost forever, which is why publishers combine direct-sold advertising deals with programmatic channels and why unsold remnant inventory ends up in open auctions at lower prices. Key health metrics are fill rate and sell-through rate.
Not all inventory is equal. Location on the page, viewability, audience, content and format all change the price. Buyers also describe inventory by channel ("CTV inventory") or quality ("premium inventory"). The word is also what fraudsters counterfeit: Domain spoofing and app spoofing pass off junk inventory as famous brands' supply.
Think of it like this
Ad inventory is like seats on a flight: once the plane takes off, an empty seat earns nothing, so airlines sell some in advance and discount the rest at the last minute.
An example
A podcast network with 400 episodes a month, each carrying two pre-roll and two mid-roll spots across 50,000 downloads, has roughly 80 million audio ad impressions of inventory per month.
Related terms
Ad slot (ad unit)
An ad slot (or ad unit) is a specific, defined place on a page, app screen or video stream where an ad can be shown, with its own size, format and rules.
Fill rate
Fill rate is the percentage of ad requests that are actually filled with an ad, calculated as impressions served divided by ad requests, multiplied by 100.
Sell-through rate
Sell-through rate is the percentage of a publisher's available inventory that has been sold, usually through direct or guaranteed deals, over a given period.
Remnant inventory
Remnant inventory is ad space a publisher has not sold through direct deals, which is then offered through programmatic channels, networks or house ads, usually at lower prices.
Publisher
A publisher is any owner of content or a digital property, such as a website, app, streaming service, podcast or screen network, that earns money by selling ad space to advertisers.