Short for click-through rate, CTR is the number of clicks an advertisement gets, divided by the advertisement's impressions. It is an important tool in measuring the success of an online advertising campaign. When an advertiser presents an advertisement to the user (in a web page, an e-mail, or in an app), that presentation is referred to as an impression. Then, if the user interacts with the advertisement by clicking on it, they are brought to the advertiser's website — this is called "clicking through" the advertisement.
The clickthrough rate is calculated as number of clickthroughs divided by the number of impressions. To express this number as a percentage, multiply it by one hundred. For example, if a page had 3,838 impressions and 31 clicks it would have a rounded CTR of 0.81% ((31 / 3,838) * 100 = 0.807 or 0.81%).