What is impression share?

An impression is one showing of your ad. Every time someone searches, an auction decides which ads appear. Your ad can be in the running and still not be shown.

Google's formula is impressions divided by total eligible impressions. It counts the auctions where your ad showed and the auctions where it was competitive enough to show. Google estimates eligibility itself, using factors such as targeting settings, approval status and quality. So the denominator is Google's estimate, not a count you can check.

You may also see it called search impression share, or simply IS.

Low impression share means missed chances. That is the obvious reading. Sometimes it is right. If a profitable campaign is held back by budget, more budget may bring more sales.

But the number measures presence, not profit. Raising it costs money, and the last slice can cost more per sale than the first.

It can also push you to pay for sales that would have come anyway. On your own brand terms, shoppers who search your name often reach you through the organic result as well. That is cannibalisation: paid activity taking credit for sales that would have arrived by another route. A high impression share on brand terms can look like control while it is mostly cost.

The question is not "can we show on every search?" It is "what does the last slice cost per sale, and is that sale extra?"

Our view: Impression share is a diagnostic. Read it next to margin and a test, not instead of them.