Ad capping is a technique used when deploying an online campaign via an ad server to define the maximum number of times a given ad can be displayed to the same individual on the same website or across a network of websites. Its goal is to limit the repetition of ads for each person exposed to the campaign. For example, a capping of 5 for 7 days means that an individual will see a maximum of 5 times the same ad in a week. This practice aims to prevent fatigue due to overexposure to advertising and to optimize return on investment by avoiding payment for advertising contacts that have become less effective.

A 2019 study conducted with Tradelab by La Poste Group revealed that advertising effects become saturated after 7 impressions. Capping can also influence the quality of visits following clicks. In the past, for intrusive ad formats that are now uncommon, such as transparent flash ads on website homepages, capping was often limited to a single exposure. In these cases, the visitor only saw the advertisement once, even if they returned to the site or refreshed the page. This type of ad capping was imposed by the website to protect its audience from these intrusive formats.

Ad capping can be defined on a time-based or session-based basis, which influences the number of times an ad is displayed. Generally, limiting ad repetition leads to an increase in the cost of display advertising, typically around 10 to 15%. This practice is particularly important in the context of retargeting campaigns. For example, to maximize the reach of a campaign, ad capping can be used strategically.