What is absolute top impression share?
Absolute top impression share is the proportion of your eligible Google Search impressions that appeared in the very first ad slot, above the organic results. The share you did not get is split into two reported figures: lost to rank and lost to budget. It measures position, not profit, and a campaign can hold the top slot consistently at a cost per sale the business cannot afford.
Google reports three related numbers per campaign or ad group: absolute top impression share, impression share lost to rank, and impression share lost to budget. Together they account for the impressions you were eligible for, so they answer a narrow question well: of the auctions you could have entered, how often did you take the most prominent position, and when you did not, what stopped you.
The split between the two loss figures is the part worth acting on, because they have different fixes and only one of them costs money. Lost to budget means the campaign ran out of money before the auction happened, and more budget genuinely recovers those impressions. Lost to rank means you were in the auction and lost it, which more budget does nothing about: that one moves on bid, expected click-through rate, ad relevance and landing page experience. Reading a rank loss as a budget problem is the single most common way a budget increase gets approved and then does nothing.
One reporting detail catches people out in spreadsheets. Google does not publish exact values at the extremes: anything below 10% is reported as less than 10%, and anything above 90% as more than 90%. Those are censored bounds, not measurements, so converting them to 0.0999 and 0.9001 to make a chart work invents precision that was never there. A campaign moving from 'more than 90%' to 'more than 90%' has told you nothing, and a target expressed as a decimal inside those bands cannot be verified.