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Google Analytics 4 explained: the G- measurement ID

NyXi

NyXi

7 min read

What Google Analytics 4 is, what a G-XXXXXXXXXX measurement ID identifies, how the Google tag and GA4 event tags differ, and how to tell if yours is set up correctly.

If you have run Tag Explorer on your site, Google Analytics 4 is almost certainly in the results, sitting under a heading like G-AB12CD34EF. That code is the one piece of the setup worth understanding, because it decides where every visit to your site gets recorded. Paste your page into Tag Explorer to see this tag on your own site.

Google Analytics 4, usually shortened to GA4, is Google’s current analytics platform. It records what people do on your site: which pages they open, where they arrived from, which forms they submit, what they buy. It replaced Universal Analytics, which no longer collects anything at all.

What it actually does

GA4 is an event recorder. Every meaningful thing that happens on your site is an event with a name and some details attached: page_view when a page opens, click when someone follows a link out, form_submit, purchase, and whatever you have configured beyond that.

Those events are sent to a property, which is the container your reports are built from. A property holds one or more data streams, and a data stream is normally one website. That structure is why GA4 can report on a site and an app together, and it is also why the same business can end up with several properties nobody has audited in years.

On top of the raw events, GA4 works out sessions, traffic sources, audiences and conversions. Almost every other measurement decision you make, from ad bidding to which pages to rewrite, is downstream of whether this data is correct.

What the ID means

Every GA4 data stream has a measurement ID in the form G-XXXXXXXXXX: the letter G, a hyphen, then ten alphanumeric characters. It is not a secret, it is visible in your page source to anyone who looks, and it is the only part of the setup that decides where data lands.

You will find yours in Google Analytics under Admin, then Data streams, by opening the stream. Match that value against what a scan of your site reports. If they differ, your reports are describing someone else’s property, or an old one.

A few related codes are easy to confuse with it:

  • G-XXXXXXXXXX is a GA4 data stream.
  • GT-XXXXXXX is a Google tag destination, which can fan out to several IDs at once.
  • AW-XXXXXXXXX is a Google Ads account.
  • GTM-XXXXXXX is a Tag Manager container, which is not analytics at all: it is the box the analytics tag travels in.

The Google tag and GA4 event tags

Inside Google Tag Manager you will usually see two kinds of GA4 tag, and they do different jobs.

The Google tag loads GA4 on the page and sends the automatic page view. This is the tag type that replaced the older “GA4 Configuration” tag, so on an inherited setup you may still see the old name. There should normally be one per property, firing on every page.

A GA4 event tag sends one specific interaction and does nothing until the Google tag has loaded. A site can have many of them: one for the enquiry form, one for a phone tap, one for a checkout step. If your form submissions are missing from GA4, this is the layer to look at, often alongside a form submission event listener.

GA4 respects Consent Mode v2, the current standard for telling Google’s tags what a visitor has agreed to. Where consent rules apply to your traffic, Google’s advertising products require those signals, and a consent platform sends them on every page.

The practical effect is that GA4 may be present, correctly installed, and still recording less than you expect, because some visitors have not consented to analytics storage. That is a working setup, not a broken one. It is worth knowing before you go hunting for a fault that is not there.

What this looks like in Tag Explorer

Tag Explorer draws your page as a tree. Each Tag Manager container is a branch, the tags inside it are leaves underneath, and anything hard coded into the page HTML sits under DIRECT ON PAGE.

GA4 appears as an Analytics card, colour coded blue, with the measurement ID as the card title and the tag type as the subtitle. A GA4 event tag has no public ID of its own, so it shows the type as the title and no copy button. That is normal and not a fault.

The things worth reading on that card:

  • Where it sits. Under a container is the healthy answer. Under DIRECT ON PAGE as well as under a container is worth investigating.
  • The info icon, which opens a short definition and the Learn more link that brought you here.
  • A red attention badge and a duplicate banner, which appear when the same measurement ID is found from two sources.
  • The insights panel below the tree, which flags split tracking: one vendor reporting to two different IDs of the same family on the same page.

Two limits are worth stating plainly. The public container file never contains the human names you gave your tags, only the tag type and the tracking ID, so a scan cannot tell you that a tag is called “GA4 Quote Form”. And the tool reads the published container, so a change sitting in a Tag Manager workspace that nobody submitted will not appear. If a tag is missing from a scan and you are certain it exists, check it was published.

A scan proves GA4 is installed. It cannot prove it fired. For that, use Preview mode in Tag Manager or the Realtime report in GA4.

What to do if it is missing, duplicated or dead

Missing. No GA4 card means the page is not recording anything, and no amount of reporting will recover the gap later: analytics is not retrospective. Check the tag exists in your Tag Manager container, that its trigger covers all pages, and that the container version was actually published. Also scan a second page, because a tag fired only on the homepage is a common and easily missed fault.

Duplicated. Two cards with the same measurement ID means everything is counted twice. Keep one, and keep the one in Tag Manager: a hard coded copy in the page template is the one to remove, because it is the copy nobody remembers editing. Two cards with different measurement IDs is a different problem: the site is reporting into two properties, so pick the property you actually use, remove the other, and make sure the surviving ID is the one in your reports.

Dead or stale. GA4 itself is current, so a GA4 tag is not dead by age. What is dead is Universal Analytics sitting beside it: standard properties stopped processing data on 1 July 2023 and Analytics 360 properties on 1 July 2024, per Google’s own shutdown notice. Tag Explorer tints those cards red for exactly that reason. Remove them.

The other stale pattern is a measurement ID belonging to a property nobody can log into, usually from a previous agency. Create a property you control, install that ID, and treat the old one as gone rather than trying to recover it.

If you want to see what your own site is reporting to right now, run it through Tag Explorer and read the measurement ID off the card. If the answer is confusing, or the ID belongs to an account nobody can access, that is the kind of thing our content and SEO structure work starts by fixing, because there is no point optimising against numbers you cannot trust.

Frequently asked questions

What does a G- measurement ID look like?

A GA4 measurement ID is the letter G, a hyphen, then ten alphanumeric characters, for example G-AB12CD34EF. It identifies one GA4 data stream, which is usually one website. You can find it in Google Analytics under Admin, then Data streams, by opening the stream.

Is it a problem if I see two different G- IDs on one page?

Usually yes. Two different measurement IDs means the page is reporting into two separate GA4 properties, so neither one holds the full picture and your reports will disagree with each other. It is occasionally deliberate, for example a group property alongside a site property, but it is far more often a leftover from an old setup or an agency handover.

What is the difference between the Google tag and a GA4 event tag?

The Google tag loads GA4 on the page and sends the automatic page view. A GA4 event tag sends one specific interaction, such as a form submission or a purchase, and it relies on the Google tag having already loaded. You want one Google tag per property and as many event tags as you have interactions worth measuring.

Does Tag Explorer prove my GA4 is working?

It proves GA4 is installed on the page and shows you which measurement ID it is reporting to. It cannot prove the tag fired and the data arrived, because it reads the page rather than watching the browser. To confirm data is flowing, use the Realtime report in GA4 or Preview mode in Google Tag Manager.

I still have Universal Analytics on the site. Does it matter?

It collects nothing. Universal Analytics stopped processing data for standard properties on 1 July 2023 and for Analytics 360 properties on 1 July 2024, and the historical data is gone. A leftover Universal Analytics tag is dead weight on every page load and should be removed.

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