Set up Google Tag Manager Data Layer

When should you use a Data Layer?

• To enhance data completeness and reliability in case of advanced or custom events, such as purchases, multi-step forms, or custom event tracking.
• To achieve accurate event forwarding and data integrity in your Server-side Tracking.

What is a Data Layer in Google Tag Manager?

What is a standard e-commerce Data Layer?

Strictly speaking, there is no universal “standard” Data Layer. Each CMS or platform often has its own variation. However, the Google Analytics 4 (GA4) Data Layer structure is widely considered the standard because it is most commonly used and directly supported by Google’s official documentation. You can find Google’s recommended e-commerce data layer here: GA4 e-commerce developer documentation.

When implementing GA4, follow Google’s standard events and structure as much as possible. Instead of using GTAG, always send events to GTM with a dataLayer.push. For example:

<script>
window.dataLayer = window.dataLayer || [];
window.dataLayer.push({
'event': 'e-commerce event',
'data': 'e-commerce data'});
</script>

Data Layer structure example: purchased event

Here’s an enhanced purchase data layer example with multiple variables, including customer information and product details:

window.dataLayer = window.dataLayer || [];
window.dataLayer.push({
event: "purchase",
transaction_id: "T_12345",
value: 72.05,
tax: 3.60,
shipping: 5.99,
currency: "USD",
coupon: "SUMMER_SALE",  customer_type: "new",
user_data: {
email: "jane.doe@example.com",
phone_number: "+14155552671",
first_name: "Jane",
last_name: "Doe",
street: "123 Main Street",
city: "San Francisco",
region: "CA",
country: "US",
postal_code: "94105"
},
items: [
{
item_id: "SKU_12345",
item_name: "Stan and Friends Tee",
affiliation: "Google Merchandise Store",
coupon: "SUMMER_FUN",
discount: 2.22,
index: 0,
item_brand: "Google",
item_category: "Apparel",
item_category2: "Adult",
item_category3: "Shirts",
item_category4: "Crew",
item_category5: "Short sleeve",
item_list_id: "related_products",
item_list_name: "Related Products",
item_variant: "green",
location_id: "ChIJIQBpAG2ahYAR_6128GcTUEo",
price: 10.01,
quantity: 3
},
{
item_id: "SKU_12346",
item_name: "Google Grey Women's Tee",
affiliation: "Google Merchandise Store",
coupon: "SUMMER_FUN",
discount: 3.33,
index: 1,
item_brand: "Google",
item_category: "Apparel",
item_category2: "Adult",
item_category3: "Shirts",
item_category4: "Crew",
item_category5: "Short sleeve",
item_list_id: "related_products",
item_list_name: "Related Products",
item_variant: "gray",
location_id: "ChIJIQBpAG2ahYAR_6128GcTUEo",
price: 21.01,
promotion_id: "P_12345",
promotion_name: "Summer Sale",
quantity: 2
}
]
});

How does the Data Layer work?

The Data Layer work client to server, acting as a communication bridge between your website (or app) and Google Tag Manager. It stores information in a structured array so that GTM can easily read it and pass it on to connected platforms such as Google Analytics, Google Ads, or other third-party tools. The typical data flow is:

1. Event occurs on the website (e.g., purchase completed).
2. The event is pushed into the Data Layer.
3. GTM web container reads the Data Layer and triggers tags or variables as configured.
4. GTM triggers tags based on configured variables and event rules.
5. Event data is sent to the GTM server container via the server_container_url parameter in your GA4 tag configuration.
6. Server container processes the event and forwards or transforms data as needed for analytics, marketing, or custom endpoints.

You can then pick up these values in GTM using Data Layer Variables. For example:

• To capture the transaction value, you would use ecommerce.value.
• To capture the first item’s ID in a purchase, you would use items.0.item_id.

These variables can then be used as parameters in your GTM tags for analytics or advertising.

This Data Layer structure ensures not only the purchase event itself is tracked but also rich details about the customer and the items purchased, maximizing server-side insights.

How to configure GTM Data Layer

There are two primary methods to send information into the data layer: declaration and push.

1. Data Layer declaration

<script>
window.dataLayer = [{
event: "pageView",
value: "content"
}];
</script>

When declaring the data layer, it should always be placed above the Google Tag Manager container snippet in the <head> section. By loading it early, GTM has immediate access to the information and can trigger tags accurately without delay.
This setup works particularly well for static information such as page titles, content categories, or user IDs, which provide valuable context for event tracking and analytics.
At the same time, it is essential to initialize the data layer correctly. An improper declaration may overwrite the existing data layer object, which can cause loss of previously defined values and lead to inconsistencies in your tracking setup.

Pro and cons

✔ Provides initial page and user info before GTM loads
✔ Ensures consistent availability of static data
✘ Cannot handle dynamic events after page load
✘ Risk of overwriting data layer

2. Data Layer push

window.dataLayer = window.dataLayer || [];
window.dataLayer.push({
event: "purchase",
value: 100
});

Using a dataLayer.push() call allows you to dynamically send new information into the existing data layer after the page has loaded. This method is ideal for recording multiple events in sequence (like add to cart, checkout, or purchase) which are essential milestones in e-commerce tracking and conversion analytics.
By pushing interaction data in real time, you can capture how users behave on your site beyond static page information.
However, it is important to initialize the data layer correctly; if the push occurs too early or too late, it can lead to timing issues where events are missed or not processed properly by Google Tag Manager. This makes careful setup critical for ensuring accurate and reliable event tracking.

Pro and cons

✔ Supports dynamic, real-time events
✔ Ideal for e-commerce actions like add-to-cart and purchase
✔ Multiple events can be recorded sequentially
✘ Requires initialization before use
✘ Order of execution must be managed carefully