Client ID

What is Client ID?


Client ID is a pseudonymous identifier assigned to a browser/device to tie hits into a single visitor across multiple visits. In practice, analytics tools generate a random ID and store it in a first-party Cookie. When that browser returns, the same ID is read, letting the platform connect Pageview and Event data into a coherent user journey and Session sequence.

How Client ID works

  1. Generation — The tracker creates a UUID-like value on the first interaction.
  2. Storage — It’s persisted (typically in a _ga-style cookie) with an expiration window. Clearing cookies, using incognito, or switching devices produces a new ID.
  3. Usage — The ID is sent with each hit so reporting can calculate visitors, Pages Per Session, Conversion Rate, and attribution.

Why it matters

  • Visitor stitching: Without a stable Client ID, your unique users inflate and funnels fragment.
  • Attribution integrity: Channel Attribution and any chosen Attribution Model rely on consistent identity to connect Referrer, UTM tags, and Campaign touchpoints—including First Touch.
  • Privacy & compliance: Client IDs are pseudonymous, not personal by themselves. Consent, retention, and purpose limits still apply.

Tool notes

  • GA4 & legacy GA: Web uses a client_id; apps use an app instance identifier. Deleting the cookie resets the visitor.
  • Alternatives: Privacy-focused tools (e.g., Plausible, Matomo, Simple Analytics) may use first-party cookies, local storage, or cookieless techniques to provide approximate visitor continuity with different trade-offs.

Limitations & gotchas

  • Cross-device gaps: The same person on phone vs. laptop yields different Client IDs; advanced Cross-Device Tracking is needed to bridge that.
  • Multi-browser reality: Safari ITP and similar restrictions can shorten storage windows, raising “new visitor” counts.
  • Data pipelines: When exporting to warehouses like BigQuery, the client_id is the backbone key for joining hit-level logs and building user-journey models in Power BI or other BI tools.

In short: Client ID is the glue that connects visits into a visitor narrative—crucial for trustworthy reporting and channel effectiveness.