GA4 Custom Dimensions and Metrics Explained

What Are Custom Dimensions?

GA4 collects dozens of default dimensions (page path, device category, session source) automatically. Custom dimensions let you capture business-specific data that GA4 doesn't know about by default — things like logged-in user status, subscription tier, product category, or content author.

Event-Scoped vs User-Scoped

Event-scoped custom dimensions are attached to individual events. Use these for attributes that can change between events (e.g., article_category on a page_view event).

User-scoped custom dimensions are attached to users and persist across sessions. Use these for stable user attributes (e.g., account_type = 'premium', subscription_plan = 'pro').

Step 1: Pass the Data in Your Event

Before registering a custom dimension in GA4, you need to send it in your event data. In GTM:

`javascript

// In your dataLayer push

window.dataLayer.push({

'event': 'page_view',

'article_category': 'GA4',

'author': 'John Smith',

'is_logged_in': true

});

`

Or in your GA4 Event tag in GTM, add custom parameters:

• Parameter name: article_category
• Value: {{DLV - article_category}}

Step 2: Register in GA4

Go to Admin → Custom Definitions → Custom Dimensions → Create.

• Dimension name: Article Category (human-readable)
• Scope: Event
• Description: Category of blog article being viewed
• Event parameter: article_category (matches what you pass in code)

Custom Metrics

Custom metrics work the same way but capture numerical values — things like article_word_count, video_progress_percent, or cart_item_count.

Using Custom Dimensions in Reports

Once registered, custom dimensions appear in:

• Explorations: Build Funnel, Segment, or Free-form reports filtered by your dimension
• Custom Reports: Add as a row or column in standard reports
• Audiences: Create audiences based on custom dimension values

Limits to Know

GA4 allows:

• 50 event-scoped custom dimensions per property
• 25 user-scoped custom dimensions
• 50 event-scoped custom metrics

Plan your custom dimensions carefully — each one counts against these limits.

Practical Examples by Business Type

E-commerce: product_brand, product_category, discount_applied

SaaS: plan_type, trial_or_paid, days_since_signup

Media: article_author, content_type, publication_date

Lead Gen: lead_source_detail, form_type, service_interest