Impression share is a Google Ads metric that compares the impressions your ads received with the impressions they could have received. If your search ads showed 6,000 times and the platform estimates they were eligible for 10,000 auctions, your impression share is 60 percent.
The report splits the missing share into two buckets. Lost impression share due to budget means your daily budget ran out and the ads stopped entering auctions. Lost impression share due to rank means you were in the auction but your Ad Rank, a combination of bid and Quality Score, was too low to be shown.
Google also reports the share of your impressions that appeared in the first position above the organic results. Together these figures answer a question no other metric does: how much room is left in this market before you would have to create new demand.
Mechanics
How the number is calculated and what moves it
Eligibility is the key idea. For every search the platform decides which advertisers could have entered the auction based on their keywords, targeting, schedule and budget status. Each of those searches counts as an eligible impression, whether or not your ad was shown. The estimate is a model, not an exact count, so it is safer to read it as a trend.
Two things raise your share. More budget removes the daily cap, so ads keep serving through the evening instead of pausing at lunchtime. A higher Ad Rank, whether through bids or better quality, wins more of the auctions you already enter. Narrowing targeting also raises the share, but by shrinking the denominator rather than winning more.
Competitors change the number without you touching anything. A restaurant chain opening in your metro area with a large ad budget will push your share down on shared terms, and the drop will show up before you see it in your click counts.
Example
A worked example for a restaurant
Picture a restaurant that advertises its catering service on search. In one month the catering campaign records 2,400 impressions, and the impression share column reads 40 percent, so the platform estimates 6,000 eligible searches. Lost share to budget is 45 percent and lost share to rank is 15 percent.
The campaign converts at 3 percent of clicks and the click-through rate is 5 percent, so 2,400 impressions gave 120 clicks and roughly 4 catering inquiries. If the restaurant lifts the budget enough to recover the 45 percent lost to budget, it would reach about 5,100 impressions, 255 clicks and 8 inquiries at the same rates, roughly doubling results without changing a single keyword.
Compare that with the remaining 15 percent lost to rank. Winning it back would need higher bids or a better score, which raises the cost per click, so the budget lever comes first here.