Impression Share

Impression share is the percentage of eligible impressions your ads actually received, out of all impressions they were eligible to receive.

It's a ceiling metric: 100% impression share means you're winning every auction you're eligible for, so there's no more volume to capture without expanding targeting, keywords, or budget. Below 100%, the lost share is broken into two causes that need completely different fixes: lost due to budget (raise budget) or lost due to rank (improve Quality Score or bid higher).

A campaign with a great CPA but low impression share is often the best opportunity on the account — proven unit economics with obvious room to scale, versus a maxed-out campaign that has nowhere left to grow.