A typical journey: a customer discovers a product on mobile on the commute, then buys it on desktop at home in the evening. If measurement doesn't connect these two visits, one person becomes two — and both attribution and customer value fall apart. Here's how cross-device measurement works.
Why the journey fragments
Visitor identification rests on cookies bound to a browser and device. Mobile and desktop each have their own client_id — without something to connect them, they look like two different people.
How to connect devices
- User ID for signed-in customers. The most reliable path: when the customer signs in, you use their internal ID as
user_id, and the platform connects activity across devices. See what client_id and user_id are and the User ID docs. - Hashed identifiers. A hashed email sent to ad platforms helps them match the same person across devices.
- Longer-lived identification. The longer identity lasts, the better the chance both visits fall into one story.
Where server-side comes in
Server-side tracking helps twice over: server-set first-party cookies last longer (fewer fragmented journeys) and the container is where you reliably add user_id and hashed identifiers to an event before forwarding it.
Cross-device vs. cross-domain
Don't confuse them: cross-device is one person on multiple devices; cross-domain is one journey across multiple domains. They use similar tools but are different problems — cross-domain is covered in a separate article.
Summary
Cross-device measurement connects mobile and desktop into one customer — the key is user ID for signed-in users, hashed identifiers and long-lived identification, which server-side strengthens. Without it you overcount people and undervalue them. More in the complete guide.