Impression Share

Impression Share is the percentage of the impressions you were eligible to receive that you actually got.

You are not showing every time someone could see your ad, and Impression Share tells you how big that gap is. Google looks at how many searches your ad was eligible for, then divides how many times you actually appeared by that total. If you were eligible for a hundred impressions and showed for sixty, your Impression Share is 60 percent. The other 40 percent is demand you were entitled to compete for and missed. It is one of the clearest ways to see how much of the available market you are capturing versus leaving on the table.

What makes this metric genuinely useful is that Google splits the missing share into two named reasons. Impression share lost to budget is the demand you missed because you ran out of money before the day was done. Impression share lost to rank is the demand you missed because your Ad Rank was not high enough to show. Same symptom, a low share, but two completely different diseases with two completely different cures.

Never act on the headline number alone. A low Impression Share caused by budget and one caused by rank call for opposite responses, so raising your bid to fix a budget problem, or adding budget to fix a rank problem, just burns money without moving the number. Read the "lost to budget" and "lost to rank" lines first, then act.

The share tells you that you are missing out; the breakdown tells you why.