Impression Share Calculator — Eligible Impressions and Lost IS

Background.

Impression share answers one question: of the auctions you could have appeared in, what fraction did you actually appear in? Google Ads defines it in a single line — "Impression share = impressions / total eligible impressions" — and then declines to show you the denominator. You get percentages, never the pool. This calculator reverses that: give it your impressions and the reported share and it reconstructs the eligible pool, then converts the two lost-impression-share percentages from abstractions into counts of impressions you can put a number on.

The reconstruction matters because percentages hide scale. A campaign at 34.2% impression share on 128,400 impressions was competing in an eligible pool of about 375,439 auctions and missed roughly 247,039 of them. Split by the reported causes — 41.7% lost to budget, 24.2% lost to Ad Rank — that is about 156,558 impressions the budget cost you and about 90,856 that Ad Rank cost you. Those are different problems with different fixes, and the percentages alone do not tell you which is worth more. Here the budget loss is roughly 1.7 times the rank loss, so the arithmetic ceiling from an uncapped budget is about 284,958 impressions, up from 128,400.

That ceiling is a ceiling, not a forecast, and it is worth being blunt about why. It holds the Ad Rank loss constant, which is exactly what will not happen: spending into auctions you previously skipped changes which competitors you meet, and Google's own definition of the denominator is an estimate rather than a count — "eligible impressions are estimated using many factors, including targeting settings, approval statuses, and quality." Every derived figure on this page inherits that estimation. Treat them as well-founded magnitudes, not as ledger entries.

Two of Google's reporting behaviours are baked into the calculator because they change how the answer should be read. First, Google clamps: its own field reference states that impression share values below 10% are reported as 9.99%, and that the lost-impression-share fields are reported between 0 and 90%, with anything above shown as 90.01%. If your share is sitting at the bottom rail, the true share is lower, the true eligible pool is larger, and the eligible and missed figures here become lower bounds — the calculator says so beside the result rather than letting you read a rail as a measurement. Second, Google publishes no statement anywhere that impression share, lost to budget and lost to rank must sum to 100%. Conceptually every eligible auction either showed your ad or did not, and if it did not the cause is budget or rank; in practice the three are independently estimated and independently rounded. So this page adds them up and reports the residual instead of assuming it away. Inside ±0.15 points that is what rounding three one-decimal figures produces. Much larger, and the figures almost certainly came from different campaigns, date ranges or networks — Search and Display impression share are separate metrics that must never be mixed.

A note on terminology, because the two big search platforms have crossed wires here. Microsoft Advertising calls its equivalent report the "Share of Voice report", and its ImpressionSharePercent column is defined as "the estimated percentage of impressions, out of the total available impressions in the market you were targeting" — the same ratio under a different name. Share of voice in the wider marketing sense is a different metric entirely, measured across a category rather than inside one ad auction. If that is what you came for, use the share of voice calculator instead. Microsoft also splits its losses more finely than Google does, adding relevance, expected click-through rate and bid alongside budget and rank, so a Microsoft loss breakdown does not map one-for-one onto the two fields here.

What is impression share calculator?

Impression share is the share of the auctions you were eligible for in which your ad actually appeared. Google Ads defines it as "the percentage of impressions that your ads receive compared to the total number of impressions that your ads could get," computed as "impressions / total eligible impressions," where "eligible impressions are estimated using many factors, including targeting settings, approval statuses, and quality." The two companion metrics decompose what you missed: Search lost IS (budget) is "the percentage of time that your ads weren't shown on the Search Network due to insufficient budget," and 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." Search and Display have separate versions of all three, and there is also an exact-match variant that restricts the denominator to searches matching your keywords exactly. Microsoft Advertising publishes an equivalent set under different names — ImpressionSharePercent, ImpressionLostToBudgetPercent and ImpressionLostToRankAggPercent — plus finer loss categories for relevance, expected click-through rate and bid. None of these is an audited figure: every one of them is described by its own publisher as an estimate, and neither platform publishes a rule that the components must sum to 100%. Impression share is a competitive diagnostic, not an accounting measure, and this page treats it as one.

How to use this calculator.

1. 01Pull impressions, impression share, lost IS (budget) and lost IS (rank) for the SAME campaign, the same date range and the same network. Four figures from three different pulls will not reconcile.
2. 02Leave the mode on 'Google's reported percentages' unless you have an independently modelled eligible-impression total.
3. 03Enter the impressions you received, then the reported impression share.
4. 04Enter the two lost-impression-share percentages. If you only want the eligible pool and not the split, set both to 0 — the reconciliation line will simply tell you the components do not add up, which at that point is expected.
5. 05Read the reconciliation line before you read anything else. It tells you whether the four figures describe the same thing, and whether either of Google's reporting rails is distorting the answer.
6. 06Compare 'missed because of budget' against 'missed because of Ad Rank'. Budget losses are closed with money; rank losses are closed with bid, expected CTR, ad relevance and landing-page experience. Only one of those is a spreadsheet decision.
7. 07Treat 'impressions if budget never ran out' as an arithmetic ceiling under today's split, and re-measure after any budget change rather than trusting the projection.
8. 08If the share is 9.99% or a loss figure is 90.01%, stop treating those numbers as measurements — they are the rails Google reports at, and the real values are further out.

The formula.

There is one relation and it is read in two directions. Forwards, impression share IS = impressions ÷ eligible impressions × 100. Backwards — which is the direction that matters, because Google reports the share and hides the pool — eligible impressions E = impressions ÷ (IS ÷ 100). At the worked example's figures, 128,400 ÷ 0.342 = 375,438.5964912281 eligible impressions, and missed impressions are E − I = 247,038.5964912281. The two loss figures are then plain proportions of that same pool: 375,438.5964912281 × 0.417 = 156,557.8947368421 impressions lost to budget, and × 0.242 = 90,856.1403508772 lost to Ad Rank. Adding the budget loss back to what you won gives 284,957.8947368421 as the arithmetic ceiling if budget never bound. A worked cross-check: 34.2% + 41.7% + 24.2% = 100.1%, so the residual is −0.1 points, inside the ±0.15 tolerance this page treats as rounding rather than error. On rounding stage: every intermediate quantity is carried at full arbitrary-precision decimal width and rounding happens once, at the return boundary, to ten decimal places. The impression share is specifically NOT rounded before it is inverted to get the eligible pool: rounding 34.2% to 34% would move the reconstructed pool from 375,438.5964912281 to 377,647.0588235294, inventing 2,208 impressions of opportunity from a single dropped decimal place. Three thresholds are hard-coded and each is tested immediately below, at, and immediately above its cut-point. Ten percent is Google's floor for reporting impression share; a share strictly below it triggers the lower-bound warning, a share of exactly 10% does not. Ninety percent is Google's ceiling for reporting a lost impression share; a figure strictly above it triggers the rail warning, exactly 90% does not. And ±0.15 percentage points is the reconciliation tolerance, chosen because three metrics each rounded to one decimal place can be off by at most three times 0.05 combined; 0.15 is treated as rounding and 0.16 as a data mismatch.

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.