UTM parameters are tags you add to the end of a link, such as utm_source, utm_medium, and utm_campaign, to tell GA4 where a visit came from, by what means, and through which campaign.
GA4 fills its source, medium, and campaign dimensions with these values, and then classifies them into channels like Email, Paid Search, and Organic Social according to the rules of the default channel group.
In this post, I lay out how UTM values lead to channel classification.
Have you ever put links in a newsletter and social media ads, opened GA4, and found the numbers piling up in "Unassigned" instead of the channel you expected? When the channel report doesn't pick things up the way you want even though you added UTMs, it's hard to know where to start looking. This can happen when your UTM values don't match GA4's channel rules, so in this post I'll walk through the two together, step by step.
1. What Are UTM Parameters?
The URL builder help article explains that adding UTM parameters to referral links or ad destination URLs lets you see which campaigns sent you traffic. The main parameters are as follows.
| Parameter | Meaning | Example |
|---|---|---|
| utm_source | Where the traffic came from (referrer) | google, newsletter4 |
| utm_medium | Marketing medium | cpc, banner, email |
| utm_campaign | Campaign name (product, slogan, promo code) | spring_sale |
| utm_id | Campaign ID | Same as the ID used when uploading campaign data |
| utm_term | Paid keyword | Search keyword |
| utm_content | Distinguishes ad creatives | Distinguishing two links in the same email |
The help article recommends that you always use utm_source, utm_medium, and utm_campaign when adding parameters.
For example, a summer sale email would look like https://www.example.com/?utm_source=summer-mailer&utm_medium=email&utm_campaign=summer-sale.
If writing these by hand is a hassle, you can also generate them with the Campaign URL Builder linked from the help article.
2. Where Do UTM Values Go in GA4?
According to the traffic-source dimensions help article, utm_source goes into Manual source, utm_medium into Manual medium, and utm_campaign into Manual campaign name, and each is also split into session-scoped and first user-scoped dimensions.
In reports, you can check them with dimensions like Session source / medium and Session campaign in the Traffic acquisition report.
There are two things to know here.
- Values are case-sensitive: The help article explains that
utm_source=googleandutm_source=Googleare treated as different values. That means the row splits in two in your reports. - Google Ads auto-tagging takes priority: If you use manual tagging and auto-tagging together, classification dimensions like source and medium follow the auto-tagging values. The Google Ads help article also says that in GA4, auto-tagging takes priority over manual tagging.
So how do the source and medium values recorded this way become a "channel"?
3. What Is the GA4 Default Channel Group?
The default channel group help article describes channels as rule-based definitions of your traffic sources. Instead of a person classifying traffic one by one, it automatically sorts traffic by checking whether values like source, medium, and campaign name match predefined rules. Default channels can be viewed through three dimensions depending on what you need.
| Dimension | What it shows |
|---|---|
| Session default channel group | The channel through which a user started a new session |
| First user default channel group | The channel through which a user first arrived |
| Default channel group | The channel through which a user arrived and triggered a key event |
It's easy to understand if you think of how a post office sorts mail. If the address on the envelope is the UTM value, the default channel group is the sorting rulebook that decides which slot to put the mail in based on that address. If the address doesn't match any slot in the rulebook, the mail ends up in the "unsortable" slot. The default channel group is managed by Google, so you can't edit it directly; if you want to change the rules, you need to create a separate custom channel group.
4. Which Channels Do utm_source and utm_medium Map To?
From the same help article, here are just the frequently used rules that apply to manually tagged traffic such as UTMs.
| Channel | Conditions (summary) |
|---|---|
| Direct | Source is (direct) and medium is (not set) or (none) |
| Paid Search | Source is on the list of search sites, and medium contains cp, or medium is ppc or retargeting, or starts with paid |
| Paid Social | Source is on the list of social sites, and medium matches the same paid patterns as above |
| Display | Medium is one of display, banner, expandable, interstitial, cpm |
| Organic Social | Source is on the list of social sites, or medium is social, sm, etc. |
| Organic Search | Source is on the list of search sites, or medium is organic |
Source or medium is email, e-mail, e_mail, e mail | |
| Referral | Medium is one of referral, app, link |
| Affiliates | Medium is affiliate |
Applying these rules, the summer sale email from earlier (utm_medium=email) is picked up as Email.
If you tagged a search ad with utm_medium=cpc, it becomes Paid Search when the source is a value on the list of search sites, like google.
On the other hand, if you use a source that isn't on the list and a medium that isn't in the rules, like utm_source=summer_event&utm_medium=promotion, it doesn't match any channel's conditions and stays Unassigned.
The help article describes Unassigned as the value used when no channel rule matches the event data.
For reference, the channel definitions themselves are not case-sensitive.
Email and email both go into the Email channel, but the difference is that they show up as two separate rows in the source/medium report.
5. What Should You Watch Out for When Adding UTMs?
Here's a summary of the recommendations in the URL builder help article.
- Use lowercase consistently:
Metaandmeta, orSpringSaleandSpring_Sale, are counted as different values. - Use only one medium value per channel: If you use several names for the same channel, like
newsletter,mail, andemail, your reports get fragmented, and names not in the rules may fall into Unassigned. - Always use all three: The article recommends filling in all related parameters once you add any UTM, and notes that missing values show up as
(not set)in reports. - Match medium values to the channel rules: The help article explains that your UTM strategy directly affects the accuracy of the default channel group. It's safest to use values that are in the rules, like
email,cpc, andsocial.
In my view, the approach with the fewest mistakes is to define a table of the UTM values your team uses, and pick from that table every time you create a link.
Summary
UTM parameters fill in GA4's source, medium, and campaign values, and the default channel group classifies those values into channels according to its rules.
So using utm_medium values that are in the rules, consistently in lowercase, helps reduce traffic that falls into Unassigned.
For the full rules and lists for each channel, see the default channel group help article linked in this post.
FAQ
- Why does the Unassigned channel appear in GA4?
- It's the value GA4 assigns when source, medium, and campaign values don't match any rule in the default channel group. If you use a utm_medium value that isn't in the rules, it may be picked up as Unassigned.
- Are UTM values case-sensitive?
- Yes. Parameter values in reports are case-sensitive, so google and Google are counted separately. However, the channel classification rules are not case-sensitive.
- Do I need to add UTMs to Google Ads ads too?
- If you link Google Ads and GA4 and use auto-tagging, you can get more dimensions through the GCLID. If you use it together with manual tagging, the auto-tagging values take priority in GA4.
Related Posts
- GA4 Users, Sessions, and Engagement Rate Explained - The session concept behind the session default channel group
- Understanding Attribution Models with Google Analytics - How credit for conversions is split across channels
- How to Optimize Search Ads - Using classified channel data in ad operations