Impression Share Calculator — Eligible Impressions and Lost IS

Background.

Google Ads tells you what share of the auctions you were eligible for your ads actually appeared in, and two more shares saying why you missed the rest: budget or Ad Rank. What it never tells you is how many auctions that was. Yet the size of the pool is what turns the percentages into decisions. Losing 41.7 percent of eligible auctions to budget is a footnote in a campaign eligible for a few thousand impressions a month and a large opportunity in one eligible for hundreds of thousands.

The pool can be recovered from figures you already have, because Google defines the metric as "impression share = impressions / total eligible impressions". Dividing impressions by the share rebuilds the denominator: 128,400 impressions at a 34.2 percent share means about 375,439 eligible, and the two lost shares split the 247,039 missed into roughly 156,558 lost to budget and 90,856 lost to Ad Rank, two problems with very different fixes.

These are estimates of estimates. Google says eligible impressions "are estimated using many factors, including targeting settings, approval statuses, and quality", and its reporting fields clamp at the extremes: an impression share below 10 percent comes back as 9.99 percent, and a lost share above 90 percent as 90.01 percent. At those rails the true figures lie further out, and the calculator adds a warning. Neither Google nor Microsoft documents that impression share, lost to budget and lost to rank must sum to 100 percent, so the calculator adds them up and reports the gap rather than assuming it away.

Microsoft Advertising reports the same ratio as ImpressionSharePercent, in a report it calls Share of Voice. Share of voice in the wider marketing sense, your slice of all advertising or mentions in a category, is a different measure with its own calculator.

What is impression share calculator?

Impression share is the percentage of the impressions your ads could have received that they actually did receive. Google defines it as "the percentage of impressions that your ads receive compared to the total number of impressions that your ads could get". Two companion metrics divide up the remainder: Search lost IS (budget), "the percentage of time that your ads weren't shown on the Search Network due to insufficient budget", and Search lost IS (rank), the same for "poor Ad Rank in the auction". Display has its own versions defined the same way for the Display Network. All of them are estimates, which makes impression share a competitive diagnostic rather than an accounting figure.

How to use this calculator.

1. 01Pull impressions, impression share, lost IS (budget) and lost IS (rank) for one campaign, one date range and one network. Search and Display figures must never be mixed.
2. 02Leave the mode on Google's reported percentages unless you have an eligible-impression total from your own model or another tool.
3. 03Enter the impressions and the impression share, then the two lost-share percentages. To see only the pool, set both lost shares to 0; the reconciliation line will then report a large gap, which is expected.
4. 04Read the reconciliation line before anything else. It tells you whether the figures describe the same thing and whether either reporting rail is in play.
5. 05Compare impressions missed through budget with those missed through Ad Rank. Budget losses respond to spend; rank losses respond to bids and ad quality: expected click-through rate, ad relevance and landing-page experience.

The formula.

In reported mode the eligible pool is E = I ÷ (IS ÷ 100), where I is impressions received and IS the reported share in percent. In counts mode the relation runs forward instead: IS = I ÷ E × 100, and an eligible total smaller than the impressions received is refused, since that would mean a share above 100 percent. From E everything else follows. Missed impressions are E − I. Impressions lost to budget are E × LIS_budget ÷ 100, and to Ad Rank E × LIS_rank ÷ 100. Adding the budget loss back to I gives the arithmetic ceiling if budget had never run out. The residual is 100 − (IS + LIS_budget + LIS_rank). A residual within ±0.15 points is treated as rounding, because three figures each rounded to one decimal place can each be off by up to 0.05; a larger gap suggests the figures came from different campaigns, dates or networks. Shares below 10 percent and lost shares above 90 percent trigger the rail warnings. Because the pool comes from dividing by the share, an imprecise share moves it noticeably: reading 34.2 percent as 34 percent adds about 2,208 impressions to the pool in the example.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Google Ads Help — "About impression share" (article 2497703). Verbatim: "Impression share (IS) is the percentage of impressions that your ads receive compared to the total number of impressions that your ads could get"; "Impression share = impressions / total eligible impressions"; "Eligible impressions are estimated using many factors, including targeting settings, approval statuses, and quality." Also states that a missing value "might be due to insufficient data, such as low traffic or new keywords." Official product documentation from the platform that defines the metric; free, not paywalled. Retrieved 2026-07-29.
2. [2]Google Ads Help — "Get impression share data" (article 7103314). Verbatim: Search impression share is "the impressions you've received on the Search Network divided by the estimated number of impressions you were eligible to receive"; Search lost IS (budget) is "the percentage of time that your ads weren't shown on the Search Network due to insufficient budget"; Search lost IS (rank) is "the percentage of time that your ads weren't shown on the Search Network due to poor Ad Rank in the auction"; Display variants are defined identically for the Display Network. Also states "all impression share metrics are updated within 1–2 days." Retrieved 2026-07-29.
3. [3]Google — Search Ads 360 Reporting API, metrics field reference (v0). Documents the reported ranges this page treats as rails: search_impression_share values below 0.1 are reported as 0.0999, and search_budget_lost_impression_share and search_rank_lost_impression_share are "reported between 0-0.9", with values above 0.9 shown as 0.9001. This reference also defines search_rank_lost_impression_share as "the estimated percentage of impressions ... that your ads didn't receive due to poor Ad Rank" — an impression basis, where the Help article says "percentage of time". Retrieved 2026-07-29.
4. [4]Microsoft Advertising API — ShareOfVoiceReportColumn Value Set (Bing Ads v13 reporting service; page updated 2026-01-08). The second, independent authority used to check Google's definition. Verbatim: ImpressionSharePercent is "the estimated percentage of impressions, out of the total available impressions in the market you were targeting"; ImpressionLostToBudgetPercent is "the estimated percentage of impressions your ad did not receive due to issues with your daily or monthly budget"; ImpressionLostToRankAggPercent is "the estimated percentage of impressions your ad did not receive due to issues with your ad ranking"; "data for this column is typically updated 14-18 hours after the UTC day ends." Microsoft also warns that "if KeywordId is not specified the report will not fail, but impression share data will be inaccurate." Free, independent of Google. Retrieved 2026-07-29.