What are impression share and lost IS (budget vs. rank)?

The columns are labelled in the interface as Search lost IS (budget) and Search lost IS (rank). Display campaigns report their own Display Lost IS (budget), which Google measures over the Display Network rather than the Search Network. Google reports impression share metrics separately for each campaign type and does not aggregate them across the account.

Google defines Search impression share as the impressions you received on the Search Network divided by the estimated number of impressions you were eligible to receive. It defines the two columns beside it differently: Search lost IS (budget) is the percentage of time your ads were not shown on the Search Network due to insufficient budget, and Search lost IS (rank) is the percentage of time they were not shown due to poor Ad Rank. One is a ratio of impressions and the other two are percentages of time, and Google does not state anywhere in its impression share documentation that the three sum to 100 per cent. Treat the pair as two differently measured attributions of loss, not as a partition of your eligible impressions.

The two columns name different measured constraints: lost IS (budget) attributes the loss to insufficient budget, lost IS (rank) attributes it to poor Ad Rank. Google's guidance on improving impression share gives a general list of levers, increasing the campaign budget, increasing the bid, narrowing regional targeting and improving ad quality, and does not map a lever to a column. Nor does its impression share documentation describe how the budget and rank mechanisms interact. So read each column as naming the constraint Google measured. Note also that Quality Score is an aggregated estimate of overall performance in ad auctions and, in Google's words, is not used at auction time to determine Ad Rank, so it is an estimate of ad quality rather than a lever inside the auction.

One subtlety worth flagging: a rising impression share alongside flat or falling raw impressions isn't necessarily progress. Impression share is a ratio to an estimated pool rather than a fixed one: Google states that eligible impressions are estimated using many factors, including targeting settings, approval statuses and quality. The denominator can therefore move on its own, which raises the percentage without the campaign winning more of the original opportunity. Always read impression share alongside the absolute impression count, not as a standalone trend.