In web analytics, the medium is the general type of channel that brought a visitor to your site, such as organic search, paid clicks, email or a link from another website. It sits beside the source, which names the specific place: the source might be google and the medium cpc, or the source a newsletter and the medium email.
How medium works
Google Analytics 4 works out the medium for each session in one of two ways.
Automatically. When there is no campaign tagging, GA4 reads the referrer and known click IDs. A visit from Google’s ordinary results is recorded as organic. A visit from another website’s link is referral. A visit with no referrer at all, such as a typed address or a link opened from some apps, is recorded as (none) and grouped as direct traffic. A click from a linked Google Ads account is recorded as cpc through auto-tagging.
From UTM tags. If the link includes a utm_medium parameter, GA4 uses that value instead. A link in your monthly email might end in ?utm_source=newsletter&utm_medium=email&utm_campaign=october_offer, and the session will show the medium as email.
GA4 then uses the source and medium together to place the session into a channel in the default channel group, such as Paid Search, Organic Social or Email. Those rules expect particular medium values. If your medium is something GA4 does not recognise, the session can end up as Unassigned.
Why it matters
Medium is how you tell whether money and effort are paying off by type of activity. If a small UK retailer runs Google Ads, posts on Instagram, sends a weekly email and relies on a few local directory listings, the medium is what separates those streams in the reports. When the values are messy, email traffic hides inside referral, paid social looks like organic social, and budget decisions are made on a distorted picture.
It also affects attribution. Key events are credited to channels, and channels are built from source and medium. A clean medium makes it possible to say with some confidence which activity is producing enquiries.
Common mistakes
- Putting the platform in the medium. utm_medium=facebook belongs in the source. The medium for a paid Facebook ad is something like paid_social or cpc.
- Inconsistent capitals and spellings. Email and email appear as two rows, and a home-made value such as newsletter in the medium falls outside the Email channel rules and can land in Unassigned.
- Tagging internal links. A UTM on a banner that points to another page of your own site starts a new session with the wrong medium and wipes the real origin.
- Leaving paid social untagged. Without UTMs, a paid Meta click often looks identical to an organic one.
- Inventing a new medium for every campaign. Campaign names belong in utm_campaign.
How to act on it
Write a short list of allowed medium values and share it with everyone who creates links: for example cpc, paid_social, organic_social, email, referral, affiliate and display. Use lower case only. A value GA4’s rules do not know, such as qr for printed codes, will show as Unassigned unless you add it to a custom channel group. Build tagged links with a consistent tool such as this UTM link builder rather than typing them by hand.
Then check GA4’s Traffic acquisition report with the session source / medium dimension. Look for one-off medium values, misspellings and anything sitting in Unassigned. Fix the links at the source; GA4 does not rewrite data that has already been collected.
If your channels still do not add up, sorting out tagging and channel rules is part of the groundwork I do in performance marketing before any budget is moved.