How Search Lost Impression Share (Budget) Works
Google estimates how often budget constraints kept the campaign from serving. The metric is reported as a percentage and is distinct from loss caused by Ad Rank.
A Simple Example
If a profitable campaign has 20% Search lost IS (budget), more daily budget may unlock additional eligible visibility—but not necessarily at the same marginal return.
Why Search Lost Impression Share (Budget) Matters
It helps separate a spending constraint from a competitiveness or relevance problem.
Common Misreading
It is not a promise that raising the budget will preserve CPA or ROAS. The additional auctions can be less profitable than the current ones.