Impression Share (Lost to Budget vs. Rank)
The share of possible ad impressions you actually got, and why you missed the rest.
What it means
Impression share is the percentage of times your ad could have shown that it actually did. Google also reports why you missed the rest: lost to budget (you ran out of money) or lost to rank (your Ad Rank wasn't high enough), so you can tell which problem you actually have.
Why it matters
The fix is completely different depending on the cause. If you're losing impressions to budget, spending more money can help. If you're losing them to rank, more budget won't fix it — you need a better bid, Quality Score, or ad relevance instead.
ask datanephew
Am I missing impressions because of budget or rank?
Ask datanephew to pull your impression share lost to budget versus lost to rank for a campaign — it tells you which lever is actually worth pulling.
Should I raise my budget on this campaign?
Ask datanephew to check first whether you're losing impression share to budget at all — if the loss is mostly to rank, more budget alone won't help.