Impression Share vs Search Lost IS: What's the Difference?

Impression Share (IS) is the percentage of eligible impressions your ads actually received, out of every impression they were eligible to show for, at the keyword, ad group, campaign, or account level. A campaign sitting at 60% impression share is showing up for roughly six out of every ten searches it could have appeared for.

Search Lost IS splits the missing 40% into two separate reasons, reported as Search Lost IS (Budget) - impressions missed because the campaign ran out of budget before the eligible auctions ended - and Search Lost IS (Rank) - impressions missed because the ad's rank, driven by bid and quality, wasn't competitive enough to show.

The split matters because the fix is different for each. High Lost IS (Budget) usually means the campaign is already working and simply needs more spend to capture demand it's proven it can convert - a low-risk case for a budget increase. High Lost IS (Rank) means more budget won't help at all; the constraint is bid, Quality Score, or ad relevance, and money spent without fixing that first is likely to be wasted on the same losing auctions. Reading impression share without checking which Lost IS reason is driving it is the most common way advertisers ask for - or approve - a budget increase that doesn't actually move the number.