Impression share is the percentage of impressions an advertiser's ads received out of the total number they were eligible to receive. Google Ads defines it with a single formula: impressions divided by total eligible impressions, according to its help centre. A campaign that showed 4,000 times when it could have shown 10,000 times has an impression share of 40%. It exists because raw impression counts say nothing about the market an advertiser is missing; companion metrics explain why the rest was lost.

How the calculation works

The numerator is simple. It is the count of impressions an ad actually served. The denominator is the difficult part. Eligible impressions are not observed but estimated, using "many factors, including targeting settings, approval statuses, and quality," according to Google. The estimate covers every auction in which the ad showed, plus every auction in which it was competitive enough to show but did not.

Two reasons account for a lost impression. Lost impression share (budget) is the share of eligible impressions missed because the daily budget ran out. Lost impression share (rank) is the share missed because Ad Rank, the score Google uses to order ads, was too low. In 2007 Search Engine Land described rank as a product of cost-per-click (CPC) bid and Quality Score; Ad Rank has since absorbed more inputs. Impression share, lost to budget and lost to rank should sum to roughly 100%, which makes the three columns a diagnostic set. A campaign at 55% share, 35% lost to budget and 10% lost to rank is constrained by money rather than by bids or ad quality.

Google reports the family separately by network. Search impression share and Display impression share are distinct columns, as are their budget and rank losses. The budget-loss columns are available at campaign level only, since budgets are set there. Impression share metrics are reported for each campaign type and "aren't aggregated across the account," according to Google. Data updates within one to two days.

The extended family

Several variants narrow the denominator. Search exact match impression share divides impressions for searches that exactly matched a keyword, or were close variants of it, by the exact-match impressions available. It is unavailable for Shopping.

Search top impression share measures impressions shown among the top ads, those adjacent to the top organic results, against eligible top impressions. Search absolute top impression share does the same for the single first position. A campaign can have 90% impression share and 20% absolute top impression share: it appears nearly every time, but rarely first.

Click share applies the same logic to clicks: the estimated share of all achievable clicks an advertiser received. Google introduced it for Shopping campaigns in 2015 and extended it to Search in February 2019.

Impression share also anchors the auction insights report, which sets an advertiser beside rivals in the same auctions alongside overlap rate, outranking share, position above rate and two top-of-page rates. The report hides insights where impression share is below 10%, according to Google.

Where it applies

Google calculates impression share for Search, Display, Shopping and Performance Max campaigns. Search impression share uses the Google Search Network only. Search partners "don't distinguish between 'top' vs. 'others'" and are excluded. For Performance Max, the formula combines impressions from Search and Shopping over eligible impressions from both, so YouTube, Display, Discover and Gmail inventory is not reflected.

Microsoft Advertising reports the same concepts in its Share of Voice report. Its API columns include ImpressionSharePercent, ImpressionLostToBudgetPercent, ImpressionLostToRankAggPercent, ExactMatchImpressionSharePercent, TopImpressionSharePercent, AbsoluteTopImpressionSharePercent and ClickSharePercent, according to Microsoft Learn documentation updated September 7, 2026. Shopping campaigns get only the first three, and Dynamic Search Ads are not supported. Before April 20, 2019, Microsoft calculated absolute top impression share only for Shopping campaigns.

Amazon Ads launched top-of-search impression share on September 22, 2021, defined as the percentage of top-of-search impressions a campaign received out of those it was eligible for. It covers Sponsored Products and Sponsored Brands, with a 90-day lookback window, and can lag by 12 hours, according to Amazon's launch note.

Target impression share bidding

The metric also became a bidding objective. Google introduced Target impression share in November 2018, setting bids automatically to show ads in a chosen share of eligible auctions. It replaced two older position strategies, target search page location and target outranking share.

An advertiser picks one of three locations: absolute top, top of page, or anywhere on the results page. A percentage target follows. A 65% absolute-top target asks Google to bid so the ad appears first in 65% of eligible auctions. A maximum CPC limit caps the bids; Google warns against setting it too low. Existing bid adjustments are ignored, apart from a -100% device exclusion.

Microsoft Advertising launched its own Target impression share bidding in August 2021. "You set your budget, where you want your ads to appear, and your Target Impression Share, and Microsoft Advertising automatically sets your bids," said Kevin Salat of Microsoft, according to Search Engine Land.

Origin and evolution

Google first reported impression share in April 2007 and announced it officially in July 2007, alongside lost IS (rank) and lost IS (budget), at campaign and account level for Search only, according to Search Engine Land. The AdWords API gained the columns in May 2009.

Ad group level reporting and exact match impression share began rolling out on January 30, 2012, according to Google's agency blog. Google recalculated campaign history back to May 2011 at the same time. On October 1, 2012 it split Search and Display into separate columns, added hour-of-day segmentation and revised the accuracy of the calculation. Historical data was cut off at that date, which is why Google's Search and Display impression share data still begins in October 2012. Shopping impression share data begins in September 2013, and Shopping's budget-loss column in November 2014.

The next shift came from position. Google rolled out top and absolute top impression share in November 2018 and, on February 26, 2019, announced that average position would sunset in September 2019. Pallavi Naresh, a Google Ads product manager, called the share metrics "the best metrics to use" for advertisers optimising for position. Microsoft took a different path. It removed impression share lost to ad relevance, lost to expected CTR and lost to bid from its Share of Voice report from March 2019.

Performance Max followed a contested route. In May 2023 Ginny Marvin, Google's Ads Liaison, said there were no plans to surface impression share for Performance Max. By September 2024, at DMEXCO, Google listed impression share reporting for Performance Max among new insights features.

Why it matters for marketers

Impression share is one of the few search metrics that sizes demand rather than performance. It separates two problems that look identical in a click report: a campaign that is underfunded, and one that is losing auctions.

The metric also exposes market shifts. When Amazon withdrew from Google Shopping between July 21 and 23, 2025, its median Shopping impression share fell from about 60% to 0% in the US, from 55% in the UK and from 38% in Germany, according to analysts cited by PPC Land.

Competitive intelligence vendors build on the same data. Adthena's competitor-based diagnosis of rising costs, covered in September 2026, used auction insights metrics including impression share and outranking share as inputs.

Limitations and criticisms

The denominator is an estimate produced by the platform selling the inventory. Advertisers cannot audit how Google or Microsoft counts eligible auctions, and the definition depends on quality and targeting judgments made inside the system.

The metric is also blind to value. A 100% share on unprofitable queries is a cost, not an achievement. Target impression share bidding can push CPCs high on terms where the extra visibility buys little, which is why Google recommends a maximum CPC limit and Microsoft's own guidance urged testing on low-risk campaigns first.

Coverage is partial. Search partners are excluded. Performance Max impression share ignores most of the channels Performance Max buys. Broad matching muddies the exact-match variant further: AI Max for Search campaigns broadened about 29% of exact-match impressions in a smec dataset of 383 million impressions, published August 2026.

Amazon's figures carry their own caveat. Even its reserve product for branded top-of-search placements does not guarantee 100% impression share. "They are not guaranteeing 100% impression share. It says most of the time," said Alexander Swade, an Amazon advertising specialist.

Impression share versus adjacent terms

Share of voice is the broader, older media concept of a brand's share of total category advertising, often measured by spend. Microsoft confusingly uses the label for its impression share report.

Click share measures achievable clicks won rather than impressions won. The two can diverge sharply when an ad shows often but in low positions.

Top of page rate in auction insights is the share of an advertiser's own impressions shown at the top. Top impression share is the share of eligible top impressions won. The first describes where ads appeared; the second describes how much top-of-page opportunity was captured.

Recent developments

Google has narrowed where impression share can be planned and bought. Performance Planner stopped supporting plans built on impression share, top impression share or absolute top impression share from March 9, 2026. In October 2026 Google introduced percentage limits on Display and partner spend, and its help page stated that the Google Display Network will no longer serve on Search campaigns using Target impression share. That change arrives with the removal of the 50% backend Display budget split in February 2027. As of November 2026, Target impression share is confined to Search results inventory.

Timeline

  • April 2007: Google begins reporting impression share in AdWords
  • July 2007: Google officially launches impression share, lost IS (rank) and lost IS (budget) at campaign and account level for Search
  • May 28, 2009: Impression share columns added to the AdWords API
  • January 30, 2012: Ad group impression share and exact match impression share begin rolling out; campaign data recalculated back to May 2011
  • October 1, 2012: Separate Search and Display columns and hour-of-day segmentation; current historical data begins
  • September 2013: Shopping impression share data begins
  • November 2014: Shopping lost IS (budget) data begins
  • 2015: Click share launches for Shopping campaigns
  • November 2018: Google rolls out top and absolute top impression share and launches Target impression share bidding
  • February 11, 2019: Google begins rolling out click share for Search campaigns
  • March 2019: Microsoft removes three lost impression share columns from Share of Voice reports
  • April 20, 2019: Microsoft extends absolute top impression share beyond Shopping campaigns
  • September 2019: Google sunsets average position
  • August 2021: Microsoft Advertising launches Target impression share bidding
  • September 22, 2021: Amazon launches top-of-search impression share for Sponsored Products and Sponsored Brands
  • May 2023: Google says it has no plans to surface impression share for Performance Max
  • September 18, 2024: Google announces impression share reporting for Performance Max at DMEXCO
  • July 21-23, 2025: Amazon exits Google Shopping; its median US Shopping impression share falls from about 60% to 0%
  • October 28, 2025: Amazon introduces reserve share of voice for branded Sponsored Brands search
  • March 9, 2026: Performance Planner drops impression share based plans
  • February 2027: Display Network stops serving on Search campaigns using Target impression share

Summary

Who: Google Ads, Microsoft Advertising and Amazon Ads calculate and report impression share. Search advertisers, agencies and competitive intelligence vendors such as Adthena use it to diagnose visibility, and the platforms sell bid strategies that target it.

What: The percentage of eligible ad impressions an advertiser actually received, accompanied by metrics that attribute lost share to budget or rank, and variants for exact match, top and absolute top positions, plus click share.

When: Google began reporting impression share in April 2007 and launched it officially in July 2007. Ad group and exact match data followed in January 2012, Search and Display were separated in October 2012, top and absolute top share arrived in November 2018, Amazon added top-of-search share in September 2021, and Performance Max reporting arrived in 2024.

Where: Search results pages above all, plus Shopping, Display and Performance Max on Google, the Share of Voice report on Microsoft Advertising, and top-of-search placements on Amazon. Search partner sites are excluded from Google's calculations.

Why: Impression counts alone cannot show how much available demand an advertiser missed. Impression share expresses visibility as a proportion of opportunity and separates budget limits from auction losses, though its denominator remains a platform estimate that advertisers cannot independently verify.