What is an impression share? If you’re in business that advertises you probably already know but for those that don’t, it’s the percentage of impressions that your ad receives compared to that total number of impressions it could get. It gets this through comparing it to other ads in a similar category. It’s calculated by dividing the number of impressions gotten by the number it has available. The available impressions are determined by things such as targeting settings, approval status and quality, according to Google. There are also different types of impressions shares, such as search, exact match, display impression share, etc.