What is a GCLID?
A GCLID, Google Click Identifier, is a unique tag Google Ads automatically adds to the end of your ad's landing-page URL. It is how Google later connects a conversion on your site back to the exact click, campaign and keyword that drove it.
How it works
When auto-tagging is on, Google Ads appends ?gclid=... to your destination URL for every ad click. Your site stores it (via the conversion linker), and when the visitor converts, that GCLID is sent back so Google matches the sale to the click. It is the backbone of Google Ads conversion tracking and smart bidding.
Auto-tagging vs UTMs
The GCLID is set by auto-tagging, which carries far richer data than manual UTMs and should normally be left on. In fact, replacing it with UTMs on Google Ads throws away detail, one reason auto-tagging vs UTMs matters.
What breaks it
- Redirects that strip query strings, dropping the GCLID before the page loads.
- Auto-tagging turned off, so no GCLID is ever added.
- No conversion linker, so the GCLID is not stored and conversions lose attribution.
A missing GCLID is a common hidden cause of Google Ads clicks not matching GA4 sessions.
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