Your ad sends people to one domain. The sale happens on another: a booking engine, a checkout, a portal. Without cross-domain tracking, Google Ads loses the click at the border and GA4 counts one buyer as two users. This guide explains what Google's linker does and what to do when you do not control the second domain.
Google Ads and GA4 keep their measurement IDs in first-party cookies. A first-party cookie belongs to one domain. A page on a second domain cannot read it.
When someone clicks your ad, Google Ads stores the click information in a cookie on your landing page domain. When the conversion tag fires, it reads that cookie and sends the click back with the conversion.
Google's How Google Ads tracks website conversions page states the requirement plainly: if your conversion page is on a different domain than your landing page, use the domain linker to pass the GCLID to the conversion page. If you do not, the tag on the second domain finds no click to connect.
GA4 has the same limit. Google's cross-domain measurement article says that without it, one person who visits two root domains on the same device counts as two users and two sessions. The second session often shows your own site as the referral source, so the ad that started the journey gets no credit.
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Request a Discovery Call →Google's developer guide to cross-domain measurement describes a two-step process. The source domain adds its first-party cookie values to links that point to the destination domain. The destination domain reads them and stores them as its own first-party cookies.
The values travel in a URL parameter called _gl. Google says the linker parameter expires after two minutes, so it is added at the moment of the click, not written into your links in advance. Google also says cross-domain measurement works for all Google products, including Google Analytics, Google Ads conversion measurement and Floodlight.
For GA4, the tag on every domain must use the same Google tag ID from the same web data stream. When the setup works, the destination URL shows the _gl parameter after the click, and GA4 counts one user and one session.
Google recommends setting cross-domain measurement in the GA4 Admin interface, under the Google tag settings, in Configure your domains. Google says you can create up to 100 conditions. Steps: Set up cross-domain measurement.
For Google Ads and Floodlight tags, the Conversion Linker tag has an Enable linking across domains option and an Auto Link Domains field. Google says a container that loads a Google tag on every page does not also need a conversion linker tag.
Google's linker adds the parameter to links by default. Forms that submit to another domain need the decorate forms option turned on, and Google says that option works only when the parameter sits in the query string, not in the URL fragment.
By default the conversion linker sets its cookies on the highest-level domain it can, so blog.example.com and shop.example.com usually share them. Google's GA4 article still recommends the setup for subdomains that use a different cookie domain, because they can create self-referrals.
Most broken setups still look fine in the tag settings. The break happens in the browser, between the click and the next page load. Google's documentation names these causes:
_gl can be removed before the tag reads it.stopPropagation() can block it.Asked where cross-domain tracking breaks most often, LGG Media founder Juan Colicchio says: "Between page and domain redirects, often. Or when iframes are involved."
Both match Google's own documentation. Google's cross-domain measurement article says that when a destination page redirects, the _gl parameter can be removed before the tag reads it. Google's How Google Ads tracks website conversions page says click trackers and server-side redirects must pass the GCLID on to the landing page.
Iframes are harder. Google's linker adds its parameter when a person clicks a link, or submits a form when form decoration is on. A booking form or checkout that loads inside an iframe from another domain does not come from a click on your page, so the linker has nothing to decorate.
Google Ads help also says not to fire your tags from within an iframe, for example inside another tracking tag like Floodlight. That rule is about where you place your own tags. It does not help when the conversion itself happens inside a frame from another domain.
On what LGG checks first, Juan says: "We check whether we can track across all domains without constraints, and inject tracking scripts into iframes." Before any build, we confirm that every domain in the journey will accept our tracking with no platform limits, and that the page inside each iframe can carry it. If either answer is no, the sale has to reach Google Ads another way, as the next section explains.
The linker needs your Google tag on both sides. On a marketplace, a third-party booking engine or a checkout that will not load your tag, it cannot work. The click and the sale sit in two systems that never share a cookie.
In that case the sale has to come back to Google Ads another way. The usual route is to keep the click ID from the first visit and send the closed sale back later, through offline conversion tracking or enhanced conversions for leads. That route also lets Google Ads bid on the value of the sale, not only the fact that it happened.
This is the part of the work that depends most on your stack: your CRM, your payment processor and the platforms you sell through. We build it for each account and do not publish the wiring. Ask us to build yours, or see how we approach attribution tracking.
It lets Google Ads and GA4 treat a visit that starts on one domain and converts on another as one journey. The Google tag passes its first-party cookie values from the first domain to the second in a URL parameter, so the conversion on the second domain can be linked to the original ad click.
It is the linker parameter that Google tags add to links pointing to a linked domain. It carries the first-party measurement cookie values. Google says it expires after two minutes, so it is added at the moment of the click.
Often not, because the conversion linker sets cookies on the highest-level domain by default, so subdomains share them. Google recommends the setup when subdomains use a different cookie domain, because that can cause self-referrals in GA4.
The linker needs your Google tag on both domains, so it cannot work on a marketplace or third-party checkout that will not load your tag. Keep the click ID from the first visit and send the closed sale back to Google Ads, for example through offline conversion import or enhanced conversions for leads.
Buyers who return from a third-party checkout arrive with the processor as the referrer. Add the processor's domain to the unwanted referrals list in the GA4 Google tag settings. Google allows up to 50 conditions per data stream.
Google Ads Help: How Google Ads tracks website conversions. Google Analytics Help: Set up cross-domain measurement, Identify unwanted referrals. Tag Manager Help: Conversion linker. Google for Developers: Measure activity across multiple domains. Checked on September 29, 2026.
Linking the domains keeps the click. These guides cover what to do with it.
What the GCLID is · Offline conversion tracking · Server-side tracking · Cookieless tracking
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