Google Analytics identifies a user as unique through a combination of a unique random number and the first timestamp (i.e. the time of the first visit). This combination is called the ‘Client ID’. 

Client ID is created and assigned by Google Analytics cookie _ga.

_ga cookie is made up of the following four fields:

The third and fourth field together make, what we called the client ID:

Google Analytics cookie can exist only on the device and browser where it has been set up.

Since client ID is stored in a GA cookie, the client ID can exist only on the device and browser where it has been set up. Because of this reason, by default, Google Analytics can not identify unique users across different web browsers and devices.

In GA reports, ‘client id’ is available as a primary dimension only in the ‘User Explorer’ report:

The user explorer report, show website usage data (sessions, average session duration, bounce rate, revenue, etc) for each user, via the ‘Client ID‘ dimension :

Since the ‘Client ID’ dimension is available only in the ‘user explorer’ report, you can not use this dimension in any other GA report. That makes analyzing website usage data at the user level quite difficult.

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The only solution to this problem is to collect client id data through your own efforts and send it to GA as a custom dimension.

Since ‘client id’ is completely anonymous, it can not be used to personally identify a person (unless you choose to combine it with personally identifiable data).

Follow the steps below to send client ID to GA as a custom dimension via GTM:

Step-1: Navigate to your GA property and create a new custom dimension with user-level scope:

Step-2: Note down the index number of your custom dimension:

We will use this index number later while setting up a custom dimension in GTM.

Step-3: Navigate to your GTM account and create a new JavaScript variable with the following code:

function(){

try {

   var tracker = ga.getAll()[0];

   return tracker.get(‘clientId’);

 } catch(e) {

   return false;

 }

}

This variable returns client ID if there is no error. Otherwise, it returns false.

The ‘try’ JavaScript statement is used to test a block of code for errors. The ‘catch’ JavaScript statement is used to handle the error.

Step-4: Create a new event tag that fires on the window load event. We are going to use this tag to send client ID as a custom dimension to GA:

Note: Here {{GA Default Settings}} is a Google Analytics Settings Variable I use. It returns the property ID.

Now you may ask, why I am not using the pageview tag (used to send GA data) to send the custom dimension? The ‘Return Client ID’ javascript variable does not seem to resolve on pageview event:

It seems to resolve on window load event:

In order to send client ID as custom dimension via pageview tag, I would have to fire the GA pageview tag on window load event and this is something, I don’t want to do.

Because if I fire the GA pageview tag on the window load event then it will delay the firing of the tag and can create tracking issues in the case some event fires before the pageview. So in order to avoid this problem, I am sending client ID as a custom dimension via event tag.

I do not want this event tag to impact bounce rate that is why I have set the event as a non-interaction event:

I do not want the event tag to fire, in case the client ID is not successfully retrieved. That is why I set the following condition while creating the window trigger:

Step-5: Preview and publish your GTM container.

You can now use the client ID as a primary dimension in GA custom reports:

You can now use the client ID as a secondary dimension in GA standard reports:

You can also see client ID data in ‘Top Events’ report under (Behavior > Events):