Guide 183 · Conversions · GA4 + Tag Manager
Purchase as a key event for ecommerce: the key-event side of the order, not the tag
The purchase tag belongs to the ecommerce guides. This guide covers the key-event side only: purchase is marked by GA4 on its own, and the reports read its count, its rate, and its value. It works out a week of orders as key events, and shows why the key event count and the order count are the same number only when the counting method is once per event.
After the steps, the purchase key event appears under the Key events tab in Admin, and the standard reports show it within a day. The worked example below gives the count and the rate to expect.
Download the reference: the purchase key-event reference (JSON) (757 bytes, MD5 1ee6982b4bf348502503f58c7e247995). It holds the event name, the counting method, the worked-example figures recomputed for this guide, and the checkpoints that prove the key event is recording. The figures in the file are the same figures shown on this page.
What you need
- Editor access or above at property level in GA4. Marking an event as a key event is a property setting, and the star in the Events table does not appear for a Viewer.
- A property collecting purchase events with transaction_id, value, and currency. The key-event side reads the purchase event; it does not build it.
- The counting method for purchase set to once per event, which is the default for purchase. Every purchase adds revenue, so each one must count.
The trace
- Confirm purchase is marked. Open Admin, then Data display, then Events, then the Key events tab. The purchase row is there because GA4 marks it on its own. If it is not there, the property has never collected a purchase, and the tag is the problem, not the marking.
- Read the purchase key events in the Events report. Open Reports, then Engagement, then Events. In the worked example the 28-day window shows 104 purchase key events from 96 sessions, so 8 sessions bought twice.
- Work out the session key event rate for purchase. The window shows 2,000 sessions and 96 sessions with a purchase, which is 96 / 2,000 = 4.80%. That single rate is the share of visits that bought at least once.
- Read the value beside the count. The purchase events carry a total value of 8,322.40 dollars across the 104 orders. The average order value is 8,322.40 / 104 = 80.02 dollars, rounded to two places. Value excludes tax and shipping, which travel on their own parameters.
- Check the counting method. Open the Key events tab, open the purchase row menu, and read the counting method. Once per event counts all 104 orders. Once per session would count 96, one per session, and understate the week's orders by 8.
- Compare the key event count with the shop's own order count. The shop recorded 104 orders for the same window. The two counts match exactly here because the counting method is once per event and every order sent a purchase event. A gap means a missing event, not a reporting choice.
Dead ends
DEAD END — COMPARING PURCHASE KEY EVENTS WITH REVENUE AND EXPECTING A MATCH. The key event count is orders, and revenue is money. The 104 orders and the 8,322.40 dollars are different figures for different questions, and neither replaces the other.
DEAD END — CHANGING purchase TO ONCE PER SESSION TO MATCH A GOAL. A Universal Analytics goal counted one conversion per session, but a shop counts every order. Switching purchase to once per session hides the 8 repeat orders in the worked example and understates revenue-bearing activity.
DEAD END — REBUILDING THE PURCHASE TAG TO FIX A KEY EVENT NUMBER. The tag sends the event; the marking counts it. If the purchase key event count is wrong, check the counting method and the Key events tab before touching the tag.
End state
Purchase is a default key event, and the window closes at 104 purchase key events from 96 sessions, worth 8,322.40 dollars at an average order value of 80.02 dollars.
| Figure | Value |
|---|---|
| Sessions in the window | 2,000 |
| Purchase key events | 104 |
| Sessions with a purchase | 96 |
| Session key event rate for purchase | 4.80% |
| Total purchase value | $8,322.40 |
| Average order value | $80.02 |
| Repeat orders inside counted sessions | 8 |
The session rate is 96 / 2,000 = 4.80% and the average order value is 8,322.40 / 104 = 80.02 dollars, both recomputed in Python 3 and rounded to two places. The repeat count is 104 − 96 = 8. The figures are a worked example, not a real property.
Sources
- Google Analytics developer docs — Measure ecommerce — the purchase event parameters, and value as the sum of price times quantity without tax or shipping
- Analytics Help — About key events — marking an event as a key event, the 30-event limit for a standard property, and 50 for Analytics 360
- Analytics Help — Change the counting method of key events — the once per event and once per session counting methods, and that a change applies to future data only
Checked 11 November 2026. The click paths above are the ones in Google's help and developer pages, read 11 November 2026. GA4 automatically flagging purchase as a key event, the purchase value as the sum of price times quantity without tax or shipping, and the once-per-event default are as Google documents them. No step in this guide was run inside a live Analytics property, because Admin needs a Google sign-in and this site does not sign in as anyone. The figures on this page are a worked example recomputed in Python 3 with decimal arithmetic, not numbers from a real property. Google moves menu items without warning; if a path on this page no longer matches, the linked help page is the current map.