This article is about Google Analytics 4 properties. If you're using a Universal Analytics property, refer to the Universal Analytics section of this help center.

[GA4] Measure activity across platforms

Send user IDs generated by your business to your Analytics property

The User-ID feature lets you associate your own identifiers with individual users so you can connect their behavior across different sessions and on various devices and platforms. Analytics interprets each user ID as a separate user, which provides you with more accurate user counts and a more holistic story about a user's relationship with your business.

This article explains what you can do with the User-ID feature and its limits. For implementation instructions, see Set up user IDs.

Looking for your Measurement ID to start collecting data from your website? Go here instead.

What you can do with User-ID

Compare signed in with non-signed in users

To compare the behavior of users who are signed in with the behavior of users who aren't signed in, build a comparison that uses the Signed in with user ID dimension and set the dimension value to "yes".

A comparison including the "Signed in with user ID" dimension and dimension value = yes

Report showing Users, New Users, Engagement Time, and Revenue for signed-in vs. non-signed-in users.

User exploration

The user exploration displays the users who make up an existing segment, or who make up the temporary segment that results from using other Explorations techniques. You can drill down into the list to see detailed information about individual users, including how and when that user was acquired, summary metrics for that user, and a timeline of their activities on your site or app.

Create remarketing audiences based on User-ID data

You can create remarketing audiences based on user IDs. If you've linked your Google Analytics and Ads accounts, these audiences are available in your shared library in Google Ads.

If you are using User-ID, then Analytics only includes the user identifier and the device identifier for the last device associated with each logged-in user.

If you're not using User-ID, Analytics includes all device and user identifiers in the audience information it exports to Ads.

How Analytics handles sessions with incomplete User-ID collection

Users sometimes trigger events on your site or app before signing in or after signing out. In the first instance, Analytics uses the session ID to associate that session with the user ID provided when the user signs in. In the second instance, once a user signs out, Analytics stops associating any subsequent events with that user ID.


A user starts a session with no associated user ID, and triggers Events 1 and 2. No user ID is associated with those events.

Next, the user signs in and triggers Event 3. Events 1,2, and 3 are now all associated with that user's ID.

Finally, the user signs out, and then triggers Event 4. No use ID is associated with Event 4. Events 1, 2, and 3 remain associated with that user.

Limits of User-ID

  • Guidelines and policies apply

    The User-ID feature is built for use with Google Analytics technologies. All implementations must comply with the Analytics SDK / User-ID Feature Policy.

  • User ID length

    The user IDs you send to Google Analytics must be fewer than 256 characters long.

  • User-ID doesn't apply to historical data

    Any data in your Analytics account collected and recorded prior to implementation won't be reprocessed and associated with a user ID.

  • User-ID data can't be merged across properties

    User-ID data collected in one property can't be shared or mixed with data in other properties.

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