The conversion linker is a tag in Google Tag Manager that reads the ad click information Google adds to your landing page address and saves it in first-party cookies on your own domain. Your conversion tags then read those cookies later, so a sale or enquiry can be matched back to the click that started it.
How the conversion linker works
When auto-tagging is on, a visitor arriving from a Google ad lands on a URL carrying a click identifier, most often the GCLID, or the wbraid and gbraid parameters on some iOS traffic. That parameter disappears as soon as they move to another page. The conversion linker catches it on arrival and stores it in cookies such as _gcl_aw, set on your domain rather than Google’s, which is why they survive browsers that block third-party cookies.
When the visitor converts, perhaps three pages and two days later, the Google Ads conversion tag looks for those cookies and sends the click identifier with the conversion. Without them, Google has far less to match on.
If you install the Google tag directly in your site code, it does this job itself and you do not need a separate linker. In Tag Manager you add a Conversion Linker tag and fire it on every page, usually on the Initialisation or All Pages trigger. It also has an option to carry click information across domains, which matters if checkout runs on a separate domain.
Why it matters
A missing linker is one of the quietest tracking faults. Conversions still record in some cases, so nothing looks broken, but attribution is patchier, enhanced conversions have less to work with and offline conversion imports fail to match because there is no stored GCLID to send back.
In the UK, the cookies the linker sets are advertising cookies. Under PECR they are not strictly necessary, so you need the visitor’s consent before setting them. With Google’s consent mode in place, the linker respects the ad_storage signal: when consent is refused, it does not write the cookies. You can enable URL passthrough so click information travels in page addresses instead, but treat that as a decision to take with your consent set-up in mind, not a way round the rules.
Common mistakes
- Building conversion tags in Tag Manager and forgetting the linker altogether.
- Firing the linker only on the conversion page, by which time the click identifier has gone from the URL.
- Firing it before consent is known, or outside consent mode, so advertising cookies are set without permission.
- Ignoring cross-domain set-ups, so the click information is lost when visitors move to a booking or payment domain.
How to act on it
Open your Tag Manager container and check for a Conversion Linker tag on an All Pages or Initialisation trigger. Then use preview mode: land on your site with a test GCLID in the URL, accept cookies and confirm the _gcl cookies appear in your browser’s storage. Reject cookies and confirm they do not. If the cookies appear before anyone has made a choice, the banner and the tag are out of step, and that needs fixing before any ad spend goes live.
If your checkout or booking system lives on another domain, list it in the linker’s cross-domain settings. Checking all of this is part of the tracking review in my PPC management work, because bidding is only as reliable as the click data underneath it.