Between an event and a conversion in Google Analytics 4 sits exactly one checkbox. People who do not know that measure clicks diligently for months and wonder why the conversion column stays empty in every report. People who do know it have the important part set up in ten minutes.
GA4 records plenty on its own: page views, scrolls, clicks on outbound links, internal search. Those are all events. An event only becomes a conversion once you explicitly mark it as a key event. Before that it is just a line in the data stream.
The distinction everything hinges on
Event and key event sound almost the same and are not. An event is anything a person does on your site. A key event is what you count as a success. The point of separating them: if GA4 counted every click as a success, you could never say which campaign or which page actually earns you anything. The selection is what turns noise into a signal.
In practice: open GA4, click Admin at the bottom left, then Events. There sits the list of every event already coming in. Beside each one a toggle, "Mark as key event". For a contact form that sends people to a thank-you page after submitting, it is often enough to take the page_view of that thank-you page as the condition.
The most common mistake: three too many, the one too few
I keep seeing accounts with five or six key events marked. Scroll to 90 percent, click on the phone number, newsletter visibility, video started. And then the one that matters is missing: the enquiry that actually got submitted.
The reason is human. The easy things are quick to mark, the hard one gets left. A scroll is trivial to measure. A form submit that loads no thank-you page of its own is work. So you mark the trivial ones and tell yourself you are tracking conversions now. You are tracking activity, not success.
The rule of thumb: something is only a key event if you would spend money to get it. For a submitted enquiry: yes. For a 90 percent scroll: not really. Anything that fails that test should stay an ordinary event. It is still in the data, just not counted as a success.
The form with no thank-you page
The practical sticking point on nearly every small site. Modern forms often no longer send you to a separate confirmation page; they show the success message right where you are. That removes the convenient route through page_view.
Two ways out, no developer required. First: many form tools and website builders can redirect to a URL after submitting, say /enquiry-sent. Set that up and you have your thank-you page back, and the rest is the checkbox again. Second: Google Tag Manager can listen for the form's submit event and fire a GA4 event of its own. That is the clean route, but it costs an hour or two of learning.
What you should not do: count a click on the submit button as a conversion. The button gets clicked even when the form then throws an error and nothing is sent. You end up counting attempts, not successes, and the number is systematically too high.
Why this is the foundation for everything else
Without defined key events, GA4 shows an empty conversion column in every report, and half the analysis falls away. The landing page report that tells you which entry page brings plenty of visitors but few conversions only works with that column. How to build and read that report is in reading GA4 reports.
And it connects to a question one level up: a conversion assumes people reach the form at all. If many of them leave before that, it is not a tracking problem but a page problem. How to tell the two apart is in reducing bounce rate.
The trap nobody sees coming: no backdating
One point that causes nasty surprises. Mark an event as a key event today and it applies from today. GA4 does not recalculate the past weeks. The enquiries that came in last month do not appear as conversions retroactively, even if the event was already being recorded back then.
In practice that means: the earlier you define the one key event that matters, the more history you have to compare later. Wait six months and you have thrown away six months of conversion data that cannot be recovered. It is one of the few mistakes in GA4 you cannot repair afterwards. That is why this one step belongs at the very start, not on the list for someday.
Where to start
Open Admin, Events, and see what is already coming in. Define exactly one key event: the one that genuinely earns your business something. Test it live through the realtime report by performing the action yourself and watching it appear. Only once that one number is measured cleanly is it worth thinking about more. A single, reliably counted key event is worth more than six you are not sure about.