A custom metric is a number you send to Google Analytics 4 with an event and then register, so that GA4 can total it, average it and show it in reports. Typical examples are the estimated value of a quote, the number of nights in a booking or the number of samples in a sample request.
How a custom metric works
GA4 already counts a great deal: users, sessions, events, revenue. A custom metric covers a quantity specific to your business that GA4 has no way of knowing. The site sends the number as an event parameter, for example quote_value with a value of 2400 on a quote_submitted event, and you register that parameter as a custom metric under Admin, Data display, Custom definitions.
When you register it you choose a unit of measurement, which tells GA4 how to format and treat the number:
- Standard A plain count or quantity, such as rooms or guests.
- Currency A monetary value. Send it in the currency your property reports in, which for most UK businesses is pounds sterling.
- Distance or time: for quantities such as delivery distance or seconds of video watched.
The difference from a custom dimension is simple. A dimension is a label you group by, such as “service type”. A metric is a number you add up, such as “quote value”. You often want both on the same event.
At the time of writing (October 2026), a standard GA4 property allows 50 custom metrics. Like custom dimensions, they only collect data from the moment they are registered.
Why it matters
Lead generation businesses often have no revenue figure in GA4, so every enquiry looks the same. A quote worth £300 and one worth £30,000 each count as one key event. Sending the estimated value as a number lets you compare channels by the value of what they bring in, not just the volume, which is a far better basis for deciding where to spend on Google Ads or Meta.
The same applies to other quantities that drive profit. A self-catering cottage business in the Cotswolds might track nights per booking, because a channel producing week-long stays is worth more than one producing single nights. A wholesaler might track units per trade order. None of these exist among GA4’s default metrics.
Common mistakes
- Sending text instead of a number. “£2,400” with a pound sign and a comma is text. GA4 needs 2400.
- Using a custom metric for money that belongs in the value parameter. Purchases and leads already have standard value and currency parameters that GA4 and Google Ads understand. Use those first, and add a custom metric only for quantities they do not cover.
- Counting the same thing twice. If the event fires on a confirmation page that can be refreshed, the number is added again on every reload.
- Choosing the wrong unit. A money value registered as standard will not be formatted as currency in reports.
- Registering metrics nobody reads. Every custom metric should be tied to a decision someone actually makes from it.
How to act on it
Pick the one or two numbers that would change how you judge your marketing if you could see them by channel. For most service businesses that is enquiry value; for most shops it is already covered by revenue. Agree with whoever builds the site where the number comes from, and make sure it is pushed as a plain number on the right event.
Test the event in DebugView, register the metric with the correct unit and give it a day or two to collect data. Then build an exploration with source or campaign as the dimension and your custom metric as the value. When I set this up as part of performance marketing, the value figure usually becomes the number campaigns are judged on, rather than a raw count of enquiries.