The Search Impression Share Formula
Google estimates eligibility using factors such as targeting, approval status, bids, budget, quality and Ad Rank.
If your Search impression share is 40%, your ads appeared in about four out of every ten auctions where they were eligible to appear.
It does not mean you reached 40% of every person searching for your product or service. The denominator is limited to impressions your current campaign was eligible to receive.
A Simple Example
Suppose Google estimates that your campaign was eligible for 10,000 Search impressions. Your ads received 4,000 impressions.
4,000 ÷ 10,000 = 40% Search impression share
Your campaign received 40% of its estimated eligible Search visibility.
Why Search Impression Share Matters
Search impression share helps you see whether more visibility may be available. It is most useful when read alongside the two metrics that explain why impressions were missed:
- Search lost IS (budget): the estimated share lost because the campaign did not have enough budget.
- Search lost IS (rank): the estimated share lost because Ad Rank was too low.
A budget constraint and a rank constraint call for different decisions. The first may point to spend capacity; the second may point to bids, relevance, ads, assets or landing-page quality.
Important: a higher impression share is not automatically better. More visibility is valuable only when the added searches and clicks produce qualified customers at an acceptable cost.
What Is a Good Search Impression Share?
There is no universal “good” percentage. The right level depends on the searches, profitability, competition, budget and business capacity.
A high impression share can be wasteful when the campaign attracts weak searches. A low impression share can represent an opportunity when the campaign is profitable and has room to grow.
Search Impression Share vs. Top Impression Rate
- Search impression share asks: how much eligible Search visibility did you receive?
- Search top impression rate asks: of the impressions you received, how often did your ad appear above the organic results?
- Search top impression share asks: how many of the eligible top-of-page impressions did you receive?
These columns sound similar but answer different questions.
Learn how to diagnose and improve impression share
Our full guide covers budget loss, rank loss, segmentation, common mistakes and the decisions to make before spending more.
Read the full Search Impression Share guide →