A lookback window is the stretch of time before a conversion during which an earlier click, view or impression stays eligible for credit. Set it at seven days and an advertisement seen eight days earlier counts for nothing. Set it at ninety and the same advertisement collects the sale. The buyer behaves identically. Only the accounting changes.

The control exists because exposure and purchase are separated in time, and because storage is finite. Someone who sees a video advertisement on a Tuesday may buy the following Monday, or six weeks later, or never. No system can hold every prior interaction against every future event indefinitely, so each platform draws a boundary and calls everything outside it unattributed. That boundary is among the most consequential numbers in any advertising report, and it is usually left at whatever the vendor shipped.

Two directions, two names

Google separates the concept into two settings that practitioners routinely conflate. The distinction surfaced in documentation covering how the attribution reports handle uploaded conversions: a lookback window runs backwards from the conversion, determining how far into the past an ad interaction stays eligible for credit, while a conversion window runs forwards from the interaction, determining whether a conversion is recorded at all. Google's own worked example describes a 30-day lookback considering interactions from December 31 to January 30 for a conversion logged on January 30. Any conversion without a preceding ad interaction inside its conversion window never enters Google Ads reporting in the first place.

The report-level control adjusts between 30, 60 and 90 days, opening at 30. One report, Model Comparison, adds a Default option matching the report window to the conversion window configured on each conversion action.

Where the numbers are set

In Google Ads, the click-through conversion window runs from 1 to 90 days depending on the conversion source, with 30 days applied automatically to new Search and Display conversion actions. View-through conversions default to a single day. Values sit on the conversion action, so one account can run 7 days on newsletter sign-ups and 90 on a financed purchase.

Google Analytics splits the setting by event class. Acquisition key events, meaning first_open and first_visit, default to 30 days with 7 as the alternative. Every other key event defaults to 90, with 30 and 60 available. The Admin API exposes both as named fields, acquisitionConversionEventLookbackWindow and otherConversionEventLookbackWindow. The 90-day default also governs session attribution, which is why analysts periodically discover sessions credited to campaign sources from weeks earlier and read it as a bug rather than a setting.

Campaign Manager 360 exposes the mechanism most plainly. Its API carries a LookbackConfiguration object with two integer fields, clickDuration and postImpressionActivitiesDuration, each accepting values from 0 to 90 inclusive. Entering 0 does not mean same-day: the scale starts at 0 for a 24-hour window, so 1 counts 48 hours. A 0 removes that interaction type from Floodlight tracking entirely, which is how buyers switch off view-through credit without touching creative or targeting. Values cascade from account through advertiser, campaign, placement and site, and clicks outrank impressions when both fall inside their windows.

Meta offers named combinations rather than a slider: 1-day click, 7-day click, 28-day click, 1-day view and 1-day engaged view. Amazon has moved the same way, tightening view attribution in January 2026.

Origin and evolution

Cookie-based post-click and post-impression counting arrived with the first ad servers of the late 1990s, and the 30-day convention travelled with them. The label entered mainstream marketing vocabulary as an interface control on August 24, 2011, when Google Analytics rolled out Multi-Channel Funnels with a selector marked Lookback Window at the top of each report, adjustable from 1 to 90 days and defaulting to 30. The boundary became something a marketer could drag rather than a constant buried in an ad server.

Facebook ran a 28-day click and 1-day view default for years. On January 19, 2021, ahead of Apple's App Tracking Transparency rollout, the default became 7-day click and 1-day view, and the 28-day click, 28-day view and 7-day view options left reporting. The new default applied to campaigns launched from April 26, 2021.

Privacy engineering then began capping windows from underneath. Apple's Intelligent Tracking Prevention 2.1, released on February 21, 2019, limited persistent cookies written through JavaScript to seven days. By iOS 14 the same restrictions applied across every browser on the operating system, with script-writeable storage cleared after seven days of no interaction and cookies set on link-decorated landing pages capped at 24 hours. The affiliate channel, where long cookie windows are the commercial terms rather than a reporting preference, absorbed the change by moving tracking server side.

Apple's own framework fixed the windows entirely. SKAdNetwork and its successor AdAttributionKit run three conversion windows measured from first launch, spanning days 0 to 2, 3 to 7 and 8 to 35, with postbacks delayed by a random 24 to 48 hours for the first and 24 to 144 for the others. Nothing is configurable, and nothing joins at user level.

Chrome's Attribution Reporting API proposed a hard ceiling. Source registrations carried an expiry field measured in seconds, defaulting to 30 days, which was also the maximum, with a minimum of one day. Advertisers in travel and other long-consideration categories argued in the issue tracker for 90 days to match Campaign Manager. Google declined, citing privacy properties tuned around the 30-day limit. The point became moot in October 2025, when most Privacy Sandbox technologies were retired after adoption failed.

Why the setting carries weight

The window is not only a reporting filter. It is a training signal: an account set to 7-day click teaches automated bidding to find people who convert within seven days, shaping delivery rather than describing it. Advertisers met a related consequence when it emerged that Meta counted likes, shares and saves as clicks inside those windows, meaning conversions from users who never left the platform were credited to campaigns.

Windows also decide comparability. Marketing leaders reviewing 2026 planning noted how differing attribution models, lookback windows and conversion definitions fragment measurement across platforms, leaving two systems that observe the same behaviour reporting different totals. Confidence data reflects it. Research published in October 2025 found 54.1% of marketers reporting no year-on-year change in measurement confidence, and IAB research in February 2026 recorded up to 75% of buy-side decision-makers rating attribution, incrementality and mix modelling as underperforming.

Limitations and disputes

Changes are not retroactive on most platforms, so a window edited in June splits the year into two definitions, and comparisons across the edit date compare different measurements. Google Analytics documentation on the August 2026 release was silent on the point.

Lengthening a window reliably increases reported conversions without increasing sales. Credit is redistributed, not created. Incrementality testing exists because attribution assigns credit among observed touchpoints without establishing that any of them mattered.

View-through windows draw the sharpest criticism, crediting exposure without any user action, and corrections have been unilateral rather than negotiated. Meta removed the 7-day and 28-day view-through windows from its Ads Insights APIwith effect from January 12, 2026, eliminating measurement options that upper-funnel campaigns had relied on to demonstrate value.

A gap also separates the configured from the achievable. A 90-day window on a browser that discards client-side state after seven days describes an intention rather than a measurement, implying a precision the underlying identifiers no longer support.

Disambiguation

A conversion window runs forwards from an interaction and decides whether an event is recorded. A lookback window runs backwards from the conversion and decides which prior interactions compete for credit. Platforms use the terms interchangeably; Google does not.

An attribution model distributes credit among the touchpoints inside the window. Last click, first click and data-driven models produce different answers from an identical window, and one model produces different answers from different windows.

Membership duration governs audiences rather than credit. Google Ads and Display & Video 360 enforce a 540-day maximum on Customer Match lists alongside a 100-user threshold, while Analytics audiences accept dynamic lookback conditions capped at 60 days. These decide who can be targeted, not which advertisement gets paid.

Query lookback describes how much history a reporting system scans. Amazon extended Marketing Cloud from 13 to 25 months on November 11, 2025, allowing two complete holiday seasons in one query. The term appears in regulatory work with the same shape and no advertising content, as in the five-year period specified in Montana's investigative demand to vehicle manufacturers.

Recent developments

Google Analytics removed its longest-standing fixed window on August 11, 2026, replacing the three-day engaged-view conversion window with any integer from 1 to 30 days and converting the six-step click-through preset ladder into any integer from 1 to 90. Until then, engaged-view credit ran on a separate clock from everything else in the same property.

The two largest social and video platforms are diverging. Google widened the range of configurable values while Meta narrowed the definitions available, redefining click-through attribution in March 2026 to count only link clicks for website and in-store conversions. New entrants are shipping fixed windows: OpenAI switched on view-through conversions in ChatGPT Ads on a window fixed at one day, operating independently of the configured click window and excluded from bidding and billing.

The mechanism has also crossed into commercial terms. Apple's European Union developer terms published on August 18, 2026 attach a store services commission to out-of-app sales completed within seven days of a link tap, and UK terms described in September 2026 apply the fee to return visits inside the same period without a further tap. Here the window sets an invoice rather than a report.

Timeline

  • February 1996: DoubleClick is founded in New York, bringing cookie-based post-click and post-impression counting to third-party ad serving
  • August 24, 2011: Google Analytics rolls out Multi-Channel Funnels with a Lookback Window selector adjustable from 1 to 90 days, defaulting to 30
  • February 21, 2019: Apple's Intelligent Tracking Prevention 2.1 caps script-written persistent cookies at seven days
  • September 2020: Intelligent Tracking Prevention applies across all browsers on iOS 14, with a 24-hour cap on cookies set from link-decorated landing pages
  • January 19, 2021: Facebook changes its default attribution setting to 7-day click and 1-day view, withdrawing the 28-day click, 28-day view and 7-day view options
  • April 26, 2021: The new default applies to all Facebook campaigns launched from this date
  • October 2022: SKAdNetwork 4 ships with iOS 16.1, fixing three conversion windows at days 0 to 2, 3 to 7 and 8 to 35
  • June 2024: Apple introduces AdAttributionKit at WWDC24, retaining the same three windows
  • November 11, 2025: Amazon extends the Marketing Cloud ad traffic lookback window from 13 months to 25 months
  • October 2025: Google retires most Privacy Sandbox technologies, including the Attribution Reporting API and its 30-day maximum source expiry
  • January 12, 2026: Meta removes the 7-day and 28-day view-through attribution windows from the Ads Insights API
  • March 3, 2026: Meta limits click-through attribution to link clicks for website and in-store conversions
  • August 11, 2026: Google Analytics replaces the fixed three-day engaged-view window with any integer from 1 to 30 days and the click-through preset ladder with any integer from 1 to 90 days

Summary

Who. Advertisers and agencies set the value; platforms including Google Ads, Google Analytics, Campaign Manager 360, Meta and Amazon define the permitted range; browser and operating system vendors including Apple and Google cap what any range can observe.

What. A lookback window is the period before a conversion during which an earlier click, view or impression remains eligible for attribution credit. It appears as a preset list in Google Ads, as an event-class default in Google Analytics, as the clickDuration and postImpressionActivitiesDuration fields in the Campaign Manager 360 API, and as named click and view combinations in Meta.

When. The convention dates to cookie-based ad serving in the late 1990s and became a visible control in August 2011. Privacy engineering began compressing it from February 2019, and platforms diverged sharply through 2026, with Google widening configurable ranges in August while Meta narrowed available definitions in January and March.

Where. In conversion action settings, property-level attribution settings, Floodlight configurations, ad set attribution settings, clean room query interfaces and, increasingly, in platform commercial terms.

Why. The window determines which advertisements get paid for which outcomes, and it trains automated bidding on what a successful conversion looks like. Lengthening it raises reported conversions without raising sales, shortening it hides genuine influence, and no two platforms measure the same behaviour the same way, which leaves the number responsible for much of the industry's stalled confidence in its own reporting.