← Glossary
Impression Share
Impression share is the percentage of the auctions you were eligible for in which your ad actually appeared.
Why it matters
It is the only metric that answers "would more budget buy more volume?" Without it, that question gets settled by instinct every month.
What people get wrong
Treating lost impression share as one number. It splits into two, and they mean opposite things. Lost to BUDGET means you ran out of money in auctions you were winning - a spend decision. Lost to RANK means you were outbid or outranked on quality - a bid or relevance problem. Summing them into "we lost 40%" hides which conversation to have. And high rank-loss at a healthy cost per acquisition is often correct: those auctions may simply not be worth winning.