How many key events can a GA4 property have?

Up to 30 in a standard property and 50 in Analytics 360. Google counts these on top of the events it marks by default, such as purchase. If you near the limit, unstar key events you no longer use.

Should I use Once per event or Once per session?

For purchase, Once per event: a second order in the same visit is real money. Once per session suits actions where repeats add nothing, such as the same form sent twice. Google calls Once per session legacy and recommends Once per event.

Can I mark a key event before GA4 has received it?

Yes. Google says you can mark new events as key events ahead of time, so GA4 treats them as key events once they arrive. Use + Create event with the Mark as key event toggle switched on, then trigger the action to test it.

How to set up key events in GA4, step by step

Open Admin, then Data display, then Events, and audit the Key events tab first. Keep purchase, unstar browsing events and build any missing action from its thank-you page. Set Once per event, confirm each key event in Realtime, and export paths with purchase selected alone.

By Joris van Huët, Founder & CEOUpdated 2026-10-018 min read

Usually, setting up key events in GA4 means removing more than adding. Open the Key events tab, keep purchase, unstar browsing events, and create any missing action from its thank-you page. Choose Once per event, check each key event arrives in Realtime, then use only purchase when you read or export attribution paths.

You need a Marketer role or above for most of this. Every menu path below was copied from Google's Analytics and Google Ads help pages on 1 October 2026. Where GA4 offers two routes to the same star, both are listed.

Step by step

Open the Key events tab. This is the list GA4 actually counts. Purchase sits there by default on a web store, so note every other starred event and ask whether you would pay for it. Menu path: Admin > Data display > Events > Key events tab.
Unstar what is not a result. Deselecting the star stops GA4 treating the event as a key event from now on. Data already collected keeps the old label, so write down the date. Menu path: Admin > Data display > Events > Key events tab > star icon.
Star an event that matters. If GA4 already receives the event, find it and click its star. The Events report offers the same switch beside each event's counts. Menu path: Admin > Data display > Events > Recent events > star icon. Or: Reports > Engagement > Events > Mark as key event.
Create the event from a thank-you page. Starring page_view would turn every page view into a key event, so build a new event from it instead. Use the confirmation page's address as the condition and switch on Mark as key event. Menu path: Admin > Data display > Events > + Create event > Create without code > page_view > URL.
Choose the counting method. Once per event counts every trigger, and Once per session counts one per visit. Google recommends Once per event, and a change applies to future key events only. Menu path: Admin > Data display > Events > Key events tab > (far right of the row) > Change counting method.
Decide on a value. Purchase brings its own value and currency. Give a lead-type key event a default value only if you would pay that much for the action. Menu path: Admin > Data display > Events > Key events tab > (far right of the row) > Set default key event value.
Watch it arrive. Trigger the action yourself and look for it in the Key events by event name card. For a step-by-step trace, turn on debug mode with Google Tag Assistant and open DebugView. Menu path: Reports > Realtime, or Admin > Data display > DebugView.
Check the lookback window. It sets how far back a touch can earn credit: 90 days by default for most key events, and 30 days for first_visit and first_open. Changes apply going forward. Menu path: Admin > Data display > Events > Attribution settings > Key event lookback window.
Export paths for purchase only. The paths report adds every selected key event together unless you pick one, so select purchase before you download. Google's report page calls it Key event attribution paths, under Key events, while its attribution overview lists it as Attribution paths, under Attribution. Menu path: Advertising > Key events > Key event attribution paths > key events drop-down at the top left > Share this report.
Build a Google Ads conversion, if you bid on it. Create it from the key event inside GA4. If the Google Ads account already has goals from an Analytics property, Google marks the new conversion secondary. That stops one purchase counting twice for bidding. Menu path: Advertising > Tools > Conversion management > New conversion > Next > (Google Ads account) > (key event) > Save.

A worked example

For illustration, picture a store that kept GA4's generous defaults: purchase, add_to_cart, begin_checkout and view_item all carry a star. Say the week brings 100 visitors who view a product, 20 who add to cart, 10 who begin checkout and 5 who buy, each once. In this worked example, the Key events column shows 135, and 5 of those paid.

Now unstar view_item, add_to_cart and begin_checkout. From that day, the column counts purchases only. Earlier weeks keep their old totals, so in this worked example a chart across the change drops from 135 to about 5. That cliff is pure bookkeeping.

Then the counting method. Suppose 2 of the 5 buyers place a second order in the same visit. In this worked example, Once per event counts 7 purchases and Once per session counts 5. Second orders are real money, which is why purchase belongs on Once per event.

One store's export shows why the clean-up matters most for fast buyers. On the Journeys sheet, journeys with 1 touch hold 79.5% of revenue and took 0.5 days to buy. A buyer who goes from first visit to purchase in one sitting fires every starred step on the same channel. Extra key events pile onto whichever channel brought them.

That export holds shares of revenue, and on a store revenue arrives only with purchases. A paths export with every key event selected would mix those purchases with steps that took no money.

What should I check when it looks wrong?

The star is greyed out. Your role is too low. Google's pages ask for Marketer or Editor access, depending on the action.
The new key event shows nothing. Standard reports can take up to 24 hours. A brand-new custom event sits in the Custom events table until GA4 processes it, and only then reaches the Recent events tab.
The Key events total dropped overnight. You probably unstarred something. Marking and unmarking apply from that moment, so compare periods on the same side of the change.
DebugView stays empty. Google says events don't show in debug mode if consent mode is on and the visitor declined Analytics cookies. Accept analytics cookies on your test device and try again.
Google Ads lists the conversion as secondary. Google does that when the account already has goals from an Analytics property. It stops the same purchase counting twice for bidding.
GA4 refuses another key event. A standard property allows 30 key events and Analytics 360 allows 50. Unstar one you no longer need before you add another.
The value never reaches Google Ads. The event needs a valid currency next to its value. Without one, GA4 counts the event but does not send it on.

What to do this week

Write down today's key events. In Admin, under Data display, open Events and then the Key events tab, and copy the list with today's date. Pass: you can say why each one is there. Fail: you cannot explain one, so it loses its star.
Test purchase end to end. Place a low-value real order, then open Reports, then Realtime. Pass: purchase shows in the Key events by event name card within a few minutes. Fail: it does not, so fix the purchase tag before any channel report means anything.
Export paths with purchase alone. In Advertising, open Key event attribution paths, select only purchase at the top left and download it through Share this report. Pass: the top row's Key events total sits close to your purchase count for the same dates. Fail: it is far higher, so other key events are still selected.

Check the homework. Your GA4 Attribution paths export already holds the evidence. Causality Engine reads that one file and shows what each channel caused next to what last-click gave it, in 1 to 2 minutes, for €99 once (excluding VAT), refundable within 30 days. Check the homework

Sources, 1 October 2026: Mark events as key events (Analytics Help); Create or modify key events (Analytics Help); Change the counting method of key events (Analytics Help); Events report (Analytics Help); Realtime report (Analytics Help); Monitor events in DebugView (Analytics Help); Select attribution settings (Analytics Help); Key events attribution paths report (Analytics Help); Get started with attribution (Analytics Help); Create Google Ads conversions based on Google Analytics key events (Google Ads Help)

Frequently asked questions

How many key events can a GA4 property have?
Up to 30 in a standard property and 50 in Analytics 360. Google counts these on top of the events it marks by default, such as purchase. If you near the limit, unstar key events you no longer use.
Should I use Once per event or Once per session?
For purchase, Once per event: a second order in the same visit is real money. Once per session suits actions where repeats add nothing, such as the same form sent twice. Google calls Once per session legacy and recommends Once per event.
Can I mark a key event before GA4 has received it?
Yes. Google says you can mark new events as key events ahead of time, so GA4 treats them as key events once they arrive. Use + Create event with the Mark as key event toggle switched on, then trigger the action to test it.

Go deeper: Causal attribution, explained.

Sixty-second versions of these ideas: Causality Engine on YouTube Shorts.

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