Impression share is the percentage of available impressions your ads received out of the total they were eligible for. It shows how much of the available demand you are capturing and how much is going to competitors, usually broken down by budget limits and rank limits.

Impression share answers a question founders actually care about: how much of the available demand are we missing? If you are capturing a small share and losing most of it to budget, there is room to grow by spending more. If you are losing it to rank, more budget will not help and the fix is relevance. The breakdown tells you which problem you have.

Example:

Capturing 30% of available impressions with most of the loss due to budget means demand exists that you are not funding. Losing it to rank means a relevance problem instead.

What does impression share tell you?

How much of the available demand your ads are capturing versus losing, and whether the loss is due to budget constraints or ad rank.

Why does the lost impression share breakdown matter?

Loss to budget means spending more can capture additional demand. Loss to rank means more budget will not help and relevance or bidding needs fixing.