The click-through rate (CTR) is a way of measuring the success of an online advertising campaign. A CTR is calculated by dividing the number of users that clicked on an advertisement on a web page by the number of times the advertisement was delivered (impressions). For example, if a banner advertisement was delivered 100 times (impressions delivered) and one person clicked on it (clicks recorded), the resulting CTR would be one percent.

CTR is most commonly defined as number of clicks divided by number of impressions and generally not in terms of number of persons clicking. This is an important difference, because if one person clicks 10 times on the same advertisement instead of one time, the CTR would increase in the latter definition but would stay the same in earlier definition.

 Banner ad click-through rates have fallen over time, often measuring significantly less than one percent. By selecting an appropriate advertising site with high affinity (e.g., a movie magazine for a movie advertisement), the same banner can achieve a substantially higher click-through rate. Personalized ads, unusual formats, and more obtrusive ads typically have higher click-through rates than standard banner ads.