Custom dimensions and custom metrics in Google Analytics 4 let us work with data that is not available by default in the platform.
GA4 already includes a large number of built-in dimensions and metrics, but in many cases that is not enough. If you need to analyze additional attributes or values sent with your events or user properties, you have to register them as custom definitions before you can use them properly in reports and explorations.
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What are custom dimensions in Google Analytics 4?
Custom dimensions in GA4 are additional attributes you can use to describe and analyze your data.
In practice, they are usually based on event parameters or user properties that you send to Google Analytics but that do not appear automatically as standard reporting dimensions. Once registered, they can be used in reporting and analysis. GA4 currently supports custom dimensions with different scopes, including event-scoped, user-scoped, and item-scoped definitions.
For example, if you send an event parameter that indicates the type of form submitted, the logged-in status of the user, the content author, or any other additional detail, you can register that value as a custom dimension and then use it in your analysis.
What are custom metrics in Google Analytics 4?
Custom metrics are numeric values that are not included by default in Google Analytics 4 but that you want to analyze in your reports.
Like custom dimensions, they are based on data you send to GA4. The difference is that custom metrics are designed for numerical values, such as counts, quantities, scores, or other measurable figures attached to an event. In GA4, custom metrics are event-scoped.
This means that if an event parameter contains a number you want to analyze later, you can register it as a custom metric and use it in reports and explorations once the definition becomes available.
Why custom dimensions and metrics matter
Custom dimensions and metrics are useful because they make the analysis much more flexible.
Standard dimensions and metrics are enough for many common use cases, but they do not cover every business need. If a company wants to analyze extra context that is specific to its implementation, such as subscription type, content category defined internally, product margin band, or any other non-standard value, custom definitions make that possible.
In other words, they allow GA4 to reflect the real data model of the business more accurately instead of forcing all analysis into the platform’s default fields.
How custom definitions work in GA4
To use a custom dimension or metric in Google Analytics 4, the first step is to send the underlying data to GA4, usually as an event parameter or a user property.
Once that data is being collected, you still need to register it in the GA4 interface as a custom definition. Only after that can it be used properly in reports, explorations, and some advertising workflows. Google notes that it can take 24 to 48 hours before the new custom definition becomes available in reporting.
This is an important point, because sending the parameter alone is not enough. If it is not registered as a custom dimension or metric, the data may be collected but it will not be readily available in the reporting interface in the same way.
Types of custom definitions in GA4
GA4 allows different types of custom definitions depending on the kind of data you want to register.
Custom dimensions can be user-scoped, event-scoped, or item-scoped. User-scoped dimensions are based on user properties and describe the user. Event-scoped dimensions are based on event parameters and describe a specific interaction. Item-scoped dimensions are used for e-commerce item data. Custom metrics, on the other hand, are used for numerical event parameters and are event-scoped.
This distinction matters because the scope determines how the data can be interpreted later in analysis.
When to use a custom dimension and when to use a custom metric
The choice depends on the type of data you are sending.
If the value is descriptive or categorical, such as a plan name, article type, form name, or user status, it should normally be registered as a custom dimension. If the value is numeric, such as points scored, levels completed, quantity selected, or another measurable number attached to an event, it should be registered as a custom metric.
This distinction is important because dimensions are used to describe and break down data, while metrics are used to quantify it.
How to create custom dimensions and metrics in GA4
In GA4, custom dimensions and metrics are created from the property settings.
Once the relevant parameter or user property is already being sent to Analytics, you can go to the custom definitions section and create the corresponding dimension or metric. During that process, you choose the name that will appear in reports and the parameter or property it should be based on.
After the definition is created, it will not populate historical data retroactively in the same way as if it had always been registered, so it is best to plan these definitions as early as possible when implementing a measurement setup.
Custom dimensions and metrics in Google Analytics 4 are essential when the default reporting fields are not enough for the analysis you need.
They allow you to register additional qualitative and quantitative data from your implementation and use it later in reports and explorations. When planned properly, they make GA4 much more useful and much better aligned with the real measurement needs of the business.
