About impression share
Impression share (IS) is the percentage of impressions that your ads receive compared to the total number of impressions that your ads could get.
Impression share = impressions / total eligible impressions
Eligible impressions are estimated using many factors, including targeting settings, approval statuses, and quality. Impression share data is available for campaigns, ad groups, product groups (for Shopping campaigns), and keywords.
Impression share is a good way to understand whether your ads might reach more people if you increase your bid or budget.
This article explains how impression share is calculated, and how these calculations differ for Shopping campaigns. For instructions on how to view your impression share now, learn how to Get your impression share data.
How it works overall
To estimate impression share, Google looks at the ad auctions over the course of the day, and uses internal data such as quality for you and other participants in the same ad auctions.
Impression share includes all auctions where your ad showed, and all auctions where your ad is competitive enough to show. For example, it could include auctions where your ad could show at twice its current bid, but could exclude auctions where your ad is estimated to need a 1,000% bid increase in order to appear.
Keep in mind, impression share is based on an estimate of when your ad was competitive in the auction. Small fluctuations over time don’t necessarily indicate that action is needed. Changes to your bids, quality, or Google's ad systems may change the set of auctions in which the system estimates you were competitive.
How it works for Shopping campaigns
Impression share for Shopping campaigns is calculated similarly to text ads. However, this metric considers that multiple Shopping ads from the same advertiser can show at the same time.
To avoid double counting when multiple Shopping ads from the same advertiser are in the same auction, AdWords assigns the impression and the impression opportunity to the highest ranked Shopping ad from that advertiser.