How to track across subdomains in GA4 (blog.yoursite.com)
If you have a blog, shop or app on a subdomain, a visitor moving from one to the other can get counted as a brand-new session from a referral. Good news: GA4 usually handles subdomains with almost no setup.
The good news about GA4 and subdomains
Unlike the old Universal Analytics, GA4 handles subdomains automatically as long as both use the same GA4 property and measurement ID. The first-party cookie is set at the top domain level, so moving from www to blog keeps the same session and source. Often there is nothing to configure.
When it still breaks
- Different measurement IDs on each subdomain, so each is a separate property and the handoff is lost.
- Your own subdomain showing as a referral, the classic symptom, which needs a referral exclusion or indicates separate properties.
- A subdomain on a different platform (a hosted blog or store) that injects its own tag.
How to set it up cleanly
Use one GA4 property and the same measurement ID across every subdomain, ideally deployed through one Google Tag Manager container. Confirm the subdomains are not listed as referrals, and if they are, add a referral exclusion. Note subdomains and genuinely different domains are not the same thing: crossing to a separate root domain needs cross-domain tracking, which is a bigger job.
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