How is a key event different from an ordinary event?
Your site can send events when someone views a page, follows a link, or submits a form. A key event is one of those events that you've identified as especially important. Marking it doesn't fix how the action is tracked. You still need an event that records the right action.
For example, a visitor might read your services page, open your contact form, and successfully send an inquiry. You may want to study all three steps, but the completed inquiry is a sensible first key event for a business that wants new inquiries.
Choose the action before choosing its name
Finish this sentence: 'I want to know how many times someone successfully ___.' You might fill the gap with 'sent a quote request,' 'created an account,' or 'completed an order.' Then write down what confirms success on your website.
Google provides recommended event names for common actions: generate_lead for an inquiry or information request, sign_up for creating an account, and purchase for a completed purchase. These recommended events need to be implemented; choosing a name alone doesn't make your website send them. The purchase event is marked as a key event by default.
A click on 'Contact us' only tells you someone clicked. It doesn't prove they sent a message. You can keep that click as an ordinary event while you work on measuring completed inquiries. For a publisher seeking newsletter readers, a confirmed newsletter subscription may be the action to prioritize instead.
Sources: Google Analytics: recommended events and when to send them · Google Analytics: default key events
Make sure the event describes a completed inquiry
Suppose your website has a quote form and a support form. Counting every form submission as a new sales inquiry would mix two different things. Give your developer the exact form you want to measure and the point at which the website confirms it accepted the request.
Google's lead-form tutorial shows how to create a separate event for one form rather than count all form submissions as key events. The conditions must match your own form. If the form sends information without loading a new page, ask your developer to check the success response rather than assume a page address can identify completion.
Write a short definition you can keep beside your report: 'Quote inquiry: the quote form was accepted successfully; opening the form and failed attempts don't count.' This is a hypothetical example, not a description of tracking we've tested on your site.
Sources: Google Analytics: measure a specific lead-generation form
How do you mark an existing event as a key event?
Check that you're in the correct GA4 property, the place where your website's data is collected. Open Admin > Data display > Events. In Recent events, find the event you've checked and select the star beside it. If you can't change it, ask an Editor or Administrator for help.
Marking an event affects reporting from that point onward; it doesn't turn earlier events into historical key events. Note the date you made the change. Google says to allow up to 24 hours for a newly marked key event to appear in standard reports, while Realtime can update within a few minutes.
Sources: Google Analytics: mark events and understand reporting delays
Test success, failure, and a repeated attempt
Use Google Tag Assistant to enable debug mode for your own device. Then open Admin > Data display > DebugView in GA4. DebugView shows incoming events from that test device. Follow your site's analytics consent choices; privacy settings can prevent events from appearing.
Try one successful submission using test details. Check that the chosen event arrives once. Next, try submitting with a required field missing: your completed-inquiry event shouldn't arrive. Finally, reload the confirmation page or repeat the action to check whether it sends an extra event unexpectedly. Ask your developer to fix any mismatch before using the count to judge campaigns.
You can also look for the event in Realtime's Key events by Event name card. Seeing it there confirms that GA4 recorded it as a key event. You'll still need the failed-attempt test to check that it means what you intended.
Sources: Google Analytics: enable debug mode and use DebugView · Google Analytics: verify key events in Realtime
What does the key event total actually count?
Check the event's counting method in Admin > Data display > Events, on the Key events tab. Once per event counts each time the event happens. Once per session counts it at most once within a visit, called a session. Changing this setting affects future counts, so record when you change it.
Suppose one visitor successfully sends two quote requests during the same session. With Once per event, that's two key events. With Once per session, it's one. These numbers are hypothetical. Neither method tells you how many requests became paying customers, and a once-per-session setting won't fix an event that fires on failed attempts.
In reports that offer a key event selector, choose your inquiry event when that's the question you're answering. A combined total could include signups and purchases too. Keep your event definition, test notes, and change date with the report so you know what the number means next month.
Sources: Google Analytics: key event counting methods · Google Analytics: select an individual key event in reports