An event parameter is a piece of extra detail sent with an event to describe it: which form was submitted, which file was downloaded, how much an order was worth. Each parameter is a name and a value, such as form_name set to “free quote” or value set to 249.
How event parameters work
When a tag sends an event to GA4, it can attach parameters alongside the event name. Some arrive automatically with every web event, including page_location (the full address), page_title, page_referrer and language. Others are defined by Google for particular events: the purchase event expects transaction_id, value, currency and items. The rest are custom parameters you choose yourself.
The detail that catches most people out is that GA4 collects every parameter you send but does not show custom ones in its reports until you register them. In Admin, under Custom definitions, you create a custom dimension for a text value or a custom metric for a number, pointing at the parameter name. From then on it can be used in explorations and added to reports. Registration is not retrospective, so data sent before that day does not appear in those reports, though it is kept in a BigQuery export if one was already running.
Custom parameters registered this way are usually event-scoped: the value describes that one event, not the visitor or the whole visit.
Limits apply. At the time of writing (October 2026), a GA4 event can carry up to 25 parameters, parameter names can be up to 40 characters and most values up to 100 characters, with longer allowances for page_location, page_title and page_referrer. A standard property allows 50 event-scoped custom dimensions and 50 custom metrics.
Why it matters
Parameters are what make events worth having. An event called generate_lead tells you an enquiry arrived. Add a form_name parameter and you can see whether it came from the quote form, the callback form or the newsletter box; add a service parameter and you can see whether the visitor wanted a boiler service or a full installation. A business deciding where to put next quarter’s budget needs that split, not just a total.
Parameters also carry the numbers that ad platforms bid on. The value and currency parameters on a purchase or a lead are what Google Ads reads when it optimises for conversion value. Send them wrongly and the bidding goes wrong with them.
Common mistakes
- Never registering custom parameters Then concluding the tracking is broken because nothing shows in reports.
- Registering too late. Because registration does not backfill, register at the same time as you launch the event.
- Personal data in parameters. Email addresses, names and phone numbers, often captured by accident in a page address or a form field, breach Google’s terms. Check page_location for query strings that carry them.
- Values that drift. “Quote”, “quote form” and “Quote Form ” with a trailing space report as three different values.
- Numbers sent as text. A value of “£249” will not be read as a number; send 249 with currency set to GBP.
- Unique values registered as dimensions Such as a timestamp or a session reference, which can push rows into GA4’s (other) bucket in reports.
How to act on it
- For each event in your tracking plan, list its parameters and the exact values allowed.
- Register the ones you will report on as custom dimensions or metrics, before or at launch.
- Check them in DebugView: click the event and confirm every parameter arrives with the expected value and type.
- Search page_location values in an exploration for “@” or “email=” to catch personal data leaking through addresses.
Designing events and their parameters is part of the tracking setup I do before running paid campaigns in performance marketing.