Impression share
IS / search impression share / share of voice
Impression share is how often your ads appeared as a share of the times Google estimates they could have. The denominator is an estimate, not the whole market: Google excludes auctions where you would need a huge bid increase to appear. Microsoft measures against the market it targeted, so the two are not comparable.
Impression share is usually produced to argue for a bigger budget, so it is worth knowing what it actually measures before you agree to one.
Google defines it as the percentage of impressions your ads receive compared to the total number of impressions your ads could get. Everything rests on that second figure. It is an estimate, not a count. Google says eligible impressions are estimated using factors including targeting settings, approval statuses and quality. It also draws a revealing boundary. The pool could include auctions where your ad could show at twice its current bid. It could exclude auctions where your ad would need a 1,000 percent bid increase to appear.
That single sentence undoes the most common reading of the number. A 20 percent impression share does not mean you are missing 80 percent of your market. It means you appeared in 20 percent of a modeled pool that already excluded the auctions you had no realistic chance of winning.
Two companion columns get quoted alongside it: lost impression share to budget, which Google describes as the percentage of time your ads were not shown due to insufficient budget, and lost to rank, due to poor Ad Rank. It is tempting to assume the three add up to 100 percent. Google does not claim that anywhere. Its own API documentation shows why you cannot rely on it. Impression share is reported in a range of 0.1 to 1, and any value below 0.1 is reported as 0.0999. The lost metrics are reported in a range of 0 to 0.9, and any value above 0.9 is reported as 0.9001. Three separate estimates, each with a floor or a ceiling, do not reliably sum to anything. Google also notes that lost impression share to rank is not shown on the ad groups tab if you ran out of budget at any point during the date range, so a budget problem can hide a rank problem.
Comparing platforms makes it worse. Microsoft measures against the total available impressions in the market you were targeting. Google measures against auctions where you were competitive. Those are different denominators, so the same business can hold two very different percentages truthfully at once.
In practice
A report shows 22 percent search impression share and 41 percent lost to budget, with a recommendation to triple spend. The honest reading is narrower. In the auctions Google judged you competitive for, you showed about a fifth of the time. Budget was the limit some of that time. Whether the other auctions are worth entering is a separate question the number does not answer.
Not the same as
- Market share
- It is a share of an estimated eligible pool, not of the customers in your area.
- Click-through rate
- This counts how often you appeared. That counts how often people clicked.
Why it matters to you
This is the number most often used to justify spending more, and it is the one least able to support that argument on its own. Google itself is cautious about it, noting that small fluctuations do not necessarily indicate that action is needed. One more omission worth knowing: Google says search partners are not included in impression share data.
What to ask or check
- 01What is the impression share, and what is lost to budget against lost to rank?
- 02Are we comparing a Google figure to a Microsoft one, given they use different denominators?
- 03If budget went up, what happened to inquiries, not just to impressions?
What people get wrong
That a 20 percent impression share means you are missing 80 percent of your market, and that the three impression share columns add to 100 percent. The denominator is an estimated pool that already excludes auctions you could not realistically win, and Google never claims the columns sum.
Red flags
- A budget increase argued from impression share alone, with no inquiry figure beside it.
- Google and Microsoft impression share quoted side by side as if they measured the same thing.
Where you will see it
As Search impression share, Search lost IS (budget) and Search lost IS (rank) in Google Ads, and as share of voice columns in Microsoft Advertising.