Mobile App + Web Ecommerce Tracking: Unifying the Picture

The Cross-Platform Ecommerce Challenge

Many ecommerce businesses operate both a website and a mobile app. Users may browse on the website and purchase on the app, or add to cart on the app and checkout on the website. Without unified tracking, you see two separate pictures: web GA4 data and app analytics data, with no visibility into the cross-platform customer journey.

GA4 is designed to solve this — a single property can receive data from web and multiple app platforms simultaneously.

Setting Up Cross-Platform Tracking in GA4

A GA4 property can have multiple data streams:

• Web data stream: your website
• iOS data stream: your iOS app (via Firebase SDK)
• Android data stream: your Android app (via Firebase SDK)

All three send events to the same GA4 property. Reports combine data across platforms by default.

Web Data Stream

Standard web ecommerce tracking as described in other guides — purchase events with transaction_id, value, currency, items.

iOS and Android Data Streams (Firebase)

Mobile apps use the Firebase SDK to send GA4 events. The ecommerce event structure is identical to web — same event names (view_item, add_to_cart, purchase), same parameters (transaction_id, value, items), same items array structure. The SDK handles the transmission.

// iOS Swift example (Firebase GA4)
Analytics.logEvent(AnalyticsEventPurchase, parameters: [
  AnalyticsParameterTransactionID: "T-123",
  AnalyticsParameterValue: 89.00 as NSNumber,
  AnalyticsParameterCurrency: "GBP",
  AnalyticsParameterItems: [
    [AnalyticsParameterItemID: "SKU-001",
     AnalyticsParameterItemName: "Blue Running Shoe",
     AnalyticsParameterPrice: 89.00 as NSNumber,
     AnalyticsParameterQuantity: 1 as NSNumber]
  ]
])

User Identity for Cross-Platform Stitching

GA4 can stitch cross-platform journeys together only when it knows the same person is behind the web session and the app session. This requires User ID:

1. When a user logs in on your website: call gtag('config', 'G-XXXX', {user_id: 'USER-123'})
2. When the same user is authenticated in your app: set Analytics.setUserID("USER-123") in the Firebase SDK
3. GA4 recognises the same user_id across platforms and stitches their sessions together in cross-device reports

Without User ID, GA4 cannot definitively link a web session and an app session to the same person. With User ID, the cross-device funnel analysis becomes possible.

Cross-Platform Funnel Analysis

Once web and app are in the same property with User ID implemented:

• In Explorations, create a Funnel Exploration
• Add dimension "Platform" (web/iOS/Android) as a breakdown
• See where users switch platforms in the purchase funnel

A common finding: users browse on web, add to cart on web, then switch to the app to complete purchase (or vice versa). This cross-platform cart abandonment is invisible without unified tracking.

Platform-Specific Differences in Reports

• App ecommerce events use slightly different session mechanics (Firebase SDK session handling vs web cookie sessions)
• In-app purchase events for iOS/Android apps (subscriptions, consumables) go through Apple/Google stores — these are separate from your server-side in-app purchase events
• Push notification-driven sessions require utm parameters or Firebase dynamic links for attribution

Summary

GA4 cross-platform ecommerce tracking uses one property with multiple data streams — web, iOS, Android. The Firebase SDK sends the same ecommerce events from mobile apps. User ID implementation stitches cross-platform journeys together. Without this unified view, businesses with both a website and an app are making decisions based on half their customer data.

See our Enhanced Ecommerce Tracking service for cross-platform tracking setup.

Need web and app tracking unified? Contact Adslytics.