Custom conversion events
A custom conversion event is a conversion you define from your own events rather than a built-in one. In GA4 there are two routes: mark an existing event as a key event so each occurrence counts, or create a conversion from conditions (e.g. a page_view where a parameter matches) to synthesise an event. Both let you measure goals specific to your site, with different trade-offs in precision and maintenance.
Two ways to define one
First, mark an existing event (often a custom event you send) as a key event, so every occurrence is counted as a conversion. Second, create a conversion from conditions — GA4's create-event feature can synthesise a new event when an existing event meets criteria, which you then mark as key. The first is precise if your event fires exactly on the goal; the second is useful when you cannot change instrumentation.
Choosing and maintaining
Marking a dedicated event as key is cleanest when you control the code and the event maps one-to-one to the goal. Condition-based conversions help when the signal exists only inside another event's parameters, but they are easy to mis-scope — too loose and they over-count, too tight and they miss cases. Document each definition and review it when the site changes, so conversions stay meaningful.
- Route 1: mark your custom event as a key event
- Route 2: synthesise a conversion from conditions
- Mis-scoped conditions over- or under-count
How it appears in analytics and logs
A conversion counting too broadly often comes from condition-based creation matching more than intended; one counting too narrowly may rely on an event that does not always fire.
Diagnostic use case
Measure a site-specific goal by either marking your custom event as a key event or building a conversion from conditions on an existing event.
What WebmasterID can help detect
WebmasterID lets you flag first-party events as conversions directly; GA4's two creation routes are documented here so you can map goals accurately.
Common mistakes
- Building a condition that matches more than the intended goal.
- Relying on an event that does not fire on every conversion.
- Leaving conversion definitions undocumented as the site changes.
Privacy and accuracy notes
Conversion definitions operate on event names and parameters, not identity. Keep the matching conditions to non-identifying fields. This is educational, not legal advice.
Related pages
- Conversion events (key events)
A conversion event is an ordinary event you have marked as important — a purchase, a signup, a qualified lead. GA4 now calls these 'key events'. Nothing about the event changes; marking it tells the platform to count it as a conversion and build conversion reports around it. The decisions that matter are which events to mark, and whether to count once per session or every time.
- Key events vs conversions in GA4
In 2024 GA4 renamed its measurement concept of 'conversions' to 'key events', while 'conversions' became the term used in Google Ads bidding. A key event is an event you mark as important in GA4; a conversion is how Ads counts it for optimisation. They can differ. This page explains the terminology shift, why it reduces confusion across products, and how to mark and read key events.
- Create event and modify event in GA4
Create event and modify event are GA4 admin features that let you derive new events from existing ones, or alter incoming events, entirely in the interface — no code changes or redeploys. Create event builds a new event when conditions match; modify event changes parameters or names of events as they are processed. They are powerful for fixing taxonomy after the fact, within documented limits.
- Attribution analytics
Measure goals specific to your site.
Sources and verification notes
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.