App instance ID dimension
The app instance ID dimension identifies a single installation of your app on a device — the mobile analogue of the web client ID. Firebase generates it per install. It enables per-installation analysis and joins to BigQuery exports, but it is a pseudonymous, resettable identifier: uninstalling and reinstalling, or clearing app data, produces a new app instance ID, so it must not be treated as a permanent person-level key.
What this means
App instance ID is the identifier Firebase assigns to each installation of your app on a device. It is the app-world counterpart to the web client ID: it scopes data to one install, not necessarily one person.
It appears in GA4 app reporting context and in BigQuery exports (as user_pseudo_id), making it the key for joining and deduplicating events at the installation level.
Why it resets
Because it is bound to an installation, the app instance ID changes when the app is uninstalled and reinstalled, when app data is cleared, or sometimes on a fresh device. So one person can produce several app instance IDs over time. To follow a person across reinstalls or devices you need a separately governed, consented User-ID; do not lean on app instance ID for durable identity.
- Pseudonymous, per-installation identifier
- Resets on reinstall or cleared app data
- Not a durable cross-device person key
How it appears in analytics and logs
A value identifies one app installation. A new app instance ID for a known user usually means a reinstall or cleared app data, not a new person.
Diagnostic use case
Use app instance ID to analyse behaviour per app installation and to join GA4 app data with BigQuery event exports.
What WebmasterID can help detect
WebmasterID treats installation identifiers as pseudonymous and resettable, helping you avoid mistaking reinstalls for genuinely new users.
Common mistakes
- Treating app instance ID as a permanent user ID.
- Reading reinstalls as brand-new users.
- Joining across reinstalls without a consented User-ID.
Privacy and accuracy notes
App instance ID is a pseudonymous installation identifier, not a stable cross-app personal ID. It resets on reinstall, and should be handled under your privacy policy.
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.
- Signed in with user ID dimension
The signed in with user ID dimension reports whether activity occurred while a User-ID was set — typically because the person was logged in. GA4 derives it from the presence of a developer-supplied User-ID on the session. It enables cross-device stitching of authenticated activity, but only when you have a lawful basis and a non-personal identifier, so it is governed by consent and policy, not enabled by default.
- Platform dimension: web, Android, or iOS
Platform is the dimension that records the broad surface a hit came from: web, Android, or iOS. In GA4 it is determined by the data stream the event arrived through, since a property can combine app and web streams. It is coarser than the operating-system dimension and is the right axis for comparing app versus web behaviour — but mixing app-only and web-only metrics across platforms is a frequent reporting error.
- Privacy-first analytics
Pseudonymous, resettable installation identifiers.
Sources and verification notes
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.