Summary
User-ID is the Google Analytics feature that lets you associate your own identifiers with individual customers so their behaviour connects across devices and platforms. You send the ID with each event once a user logs in, and GA4 uses it ahead of Device ID in the Reporting Identity settings. Without it, the same person browsing on mobile and desktop is counted as two separate users.
Nitin Batra is a Google Analytics 4 certified professional who implements User-ID tracking through Google Tag Manager and validates cross-device reporting identity settings for client properties.
Last updated: August 16, 2026
To associate your own identifiers with individual customers and connect their behaviour across devices and platforms, use User-ID, the GA4 feature that sends your internal logged-in user ID with every event for cross-device reporting.
- Predictive audiences
- User-ID
- Device ID
- Audience triggers
The correct answer is: User-ID
Key Takeaways
- – User-ID is the GA4 feature that lets you assign your own identifiers to individual customers and join their activity across devices and platforms.
- – You must send the user_id with events yourself through gtag, Google Tag Manager or the Firebase SDK. GA4 does not create it for you.
- – Reporting Identity in Admin controls the order GA4 uses identifiers: User-ID first, then Google signals, then Device ID, then modelling.
- – Never send email addresses, phone numbers or any personally identifiable information as the User-ID. Use a hashed or internal ID, and remember behaviour before login stays tied only to the Device ID.
How is User-ID different from Device ID in GA4?
Device ID is the client ID stored in a browser cookie or the app instance ID, so it identifies a device, not a person. If the same customer visits from a laptop and then a phone, that is two Device IDs and two users. User-ID is your own identifier for the person, so GA4 can merge those sessions into a single user journey.
Where do I set the Reporting Identity for User-ID in GA4?
Go to Admin, then Data Display or Data Settings depending on your property layout, and open Reporting Identity. Choose Blended, Observed or Device-based. Blended and Observed both give priority to User-ID when it is present, while Device-based ignores User-ID completely in reports.
Does User-ID work retroactively on data already collected?
No. GA4 only associates a User-ID with events that were sent with that parameter attached. Sessions collected before you implemented User-ID stay tied to the Device ID, so plan the rollout knowing your cross-device numbers will only improve going forward.
Can I use User-ID for audiences and BigQuery analysis?
Yes. The User-ID is available as a dimension in BigQuery exports, which is where most teams join GA4 behaviour with CRM or order data. In the interface you can build audiences and explorations on user-scoped behaviour, and the user_id itself is exposed in the User Explorer report for individual user inspection.