Click-through rate or CTR is a way of determining the effectiveness of an online advertising campaign. A CTR is obtained by dividing the number of users who clicked on an ad on a web page by the number of times the ad was delivered (impressions). CTR is most commonly defined as number of clicks divided by number of impressions. For example, if 100 people visit this webpage today, the Google Banner on the left of this paragraph blurb will receive 100 impressions. If one visitor clicks an ad within the banner that would leave a ratio of 1/100, or (after a bit of simple division) a 1% click-through rate.