Custom dimensions in GA4: setting up and auditing
Custom dimensions are the lever that turns generic GA4 tracking into an analysable data basis - your own parameters such as article category, login status or membership level end up in reports and in the BigQuery export. At the same time they are one of the most common sources of error in an audit. This page explains the setup and the typical pitfalls.
What are custom dimensions in GA4?
Custom dimensions make your own event or user parameters visible in GA4. For a parameter to appear in reports, you have to do two things: send it with the event and register it in the property (Admin > Custom definitions). If either step is missing, the dimension stays empty.
Event-scoped vs. user-scoped
- Event-scoped describes a single event - e.g.
methodon theloginevent orarticle_categoryon aview_item. - User-scoped describes the user across sessions - e.g.
membership_level- and is set as a user property. - Item-scoped describes individual e-commerce articles in the
itemsarray.
The wrong scope is a classic finding: the values then do not aggregate as expected.
How to set up custom dimensions
- Send the parameter reliably with the event (or as a user property).
- Register it under Admin > Custom definitions with the correct scope.
- Note: custom dimensions do not work retroactively - data before the
registration stays under
(not set).
Typical audit findings
| Finding | Symptom | Consequence |
|---|---|---|
| Parameter not registered | dimension missing in reports | data invisible |
| Wrong scope | values do not aggregate | misleading reports |
| Limit exceeded | 50 / 25 / 10 exceeded | new dimension cannot be created |
| High cardinality | many distinct values | GA4 groups under (other) |
| Unused slots | registered, never sent | wasted limit |
How the auditor checks this
The GA4 Auditor reads your custom definitions via the Admin API and reconciles them with the parameters actually arriving from the BigQuery export - so you can see registered but empty dimensions and sent but unregistered parameters at a glance. Check your property now.
Related topics
- GA4 audit checklist - all config checks.
- (not set) in GA4 - source analysis
- BigQuery export for GA4 - custom dimensions in the raw data export.
Frequently asked questions
- What are custom dimensions in GA4?
- Custom dimensions are user-defined dimensions with which you make your own event or user parameters visible in GA4 reports - for example article category, login status or membership level. The parameter is sent with the event and additionally has to be registered in the property so that it appears in reports.
- How many custom dimensions does GA4 allow?
- A standard GA4 property allows 50 event-scoped, 25 user-scoped and 10 item-scoped custom dimensions. GA4 360 raises these limits significantly. Registered but unused dimensions cost slots - a common clean-up item in an audit.
- What is the difference between event-scoped and user-scoped?
- Event-scoped dimensions describe a single event (e.g. method on login). User-scoped dimensions describe the user across sessions (e.g. membership_level) and are set as a user property. The wrong scope is a classic audit finding, because the values then do not aggregate in reports as expected.
- Why do my custom dimensions not appear in GA4?
- Usually three reasons: the parameter is not sent with the event, it was never registered in the property, or the data predates the registration - custom dimensions do not work retroactively. With too many distinct values, GA4 also groups them under (other) (cardinality).