User-ID dimension: identity you assign, not infer
User-ID is the dimension that carries an identifier you assign to authenticated users, enabling analytics to stitch their activity across devices and sessions. Unlike a client ID, it is identity you provide, typically only after sign-in. GA4's User-ID feature requires that the value be a non-PII pseudonymous key and that you have the appropriate consent, because it links behaviour to a known person.
What this means
When a user signs in, you can assign them a stable, pseudonymous identifier and pass it to analytics as the User-ID. From then on, their sessions and devices that share that ID are reconciled into one user, giving a truer cross-device picture than device-scoped identifiers alone.
GA4 reports a dedicated 'User-ID' reporting identity when this is configured.
Rules and limits
Google's policy is explicit: the User-ID must not contain information a third party could use to identify the individual — no emails, names, or other PII. You must also have disclosed and obtained the necessary consent, because the feature links analytics to authenticated identity.
User-ID only ever covers signed-in activity; pre-login and anonymous sessions remain device-scoped, so most properties see User-ID on a minority of traffic.
- Identity you assign at sign-in, not an inferred fingerprint
- Must be a pseudonymous key — never PII
- Requires consent; covers authenticated sessions only
How it appears in analytics and logs
A populated User-ID means the visit belonged to an identified, signed-in user. Its absence on most traffic is normal — User-ID applies only where you can authenticate, not to anonymous visitors.
Diagnostic use case
Use User-ID to unify a logged-in person's journey across devices, while passing only a pseudonymous key and honouring consent for known-user tracking.
What WebmasterID can help detect
WebmasterID favours first-party, behaviour-level measurement; where you operate a known-user identifier, it should be a pseudonymous key set with consent, not a personal detail.
Common mistakes
- Passing an email or name as the User-ID.
- Enabling User-ID without the required consent.
- Expecting User-ID to cover anonymous, pre-login traffic.
Privacy and accuracy notes
User-ID links behaviour to a known person, so it requires a lawful basis/consent and must be a pseudonymous key, never an email or name. WebmasterID-style first-party identity should follow the same non-PII rule. This is educational, not legal advice.
Related pages
- Client ID dimension: the device-scoped pseudonym
Client ID is the device-scoped pseudonymous identifier analytics generates to recognise a returning browser. In GA4 it is typically stored in a first-party cookie and underpins the user and session counts. It is not a person: clearing cookies, switching browsers, or using a new device all create a fresh client ID, which is why user counts drift upward and cross-device journeys split without a User-ID.
- Audience dimension
The audience dimension records membership in the audiences you define — groups of users meeting conditions such as 'purchasers' or 'engaged readers'. In GA4 audiences are evaluated as users meet the criteria, so membership is largely forward-looking: creating an audience does not always backfill historical members. This makes audiences a powerful segmentation tool with timing caveats that affect how you read the dimension.
- Session ID dimension: scoping events to one visit
Session ID is the dimension that ties a burst of events together into one session. In GA4 it is derived from the ga_session_id parameter set when a session_start event fires, and it pairs with the client or user ID to be unique. A session ends after a timeout of inactivity (30 minutes by default), so the same person returning later gets a new session ID — which is why session counts respond to the timeout setting.
- Privacy-first analytics
Measure known users with pseudonymous, consent-based identity.
Sources and verification notes
- Google Analytics Help — [GA4] Measure activity across platforms with User-IDDocuments User-ID requirements, including the non-PII rule.
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.