A viewable impression is an ad impression that was actually viewed by the user, within a certain period of time or a certain distance from the ad. A viewable impression is eligible for billing or attribution, depending on the standards or the guidelines of the industry, the platform, or the publisher. A viewable impression is calculated as the ratio or the percentage of ad impressions that were actually viewed by the user, within a certain period of time or a certain distance from the ad, divided by the total number of ad impressions. A viewable impression is an optional and complementary metric to served impressions, and it is used to assess the quality or the value of an ad placement, the inventory, or the targeting, beyond the quantity or the performance of the ad.