What are UTM parameters?
UTM parameters are tags appended to a URL that tell analytics tools which source, channel type and campaign a visitor came from. There are five: utm_source (facebook, newsletter), utm_medium (channel type: paid_social, email, cpc), utm_campaign (campaign name), utm_term (paid search keyword) and utm_content (ad or creative variant). Always set source, medium and campaign together; a missing parameter shows up as (not set) in GA4 reports.
How does GA4 map UTMs to channels?
GA4 assigns each session's source and medium to default channel groups (Paid Social, Paid Search, Email, Organic Social, Referral and so on) using rules Google publishes. utm_medium usually decides: mediums starting with 'paid' or containing 'cp' (cpc, cpm, paid_social) go to paid channels, 'email' to Email, 'social' to Organic Social, 'referral' to Referral and 'affiliate' to Affiliates. Source matters for paid splits: facebook + paid_social lands in Paid Social, google + cpc in Paid Search. Combinations that match no rule (e.g. source=march-digest with medium=story, or source=trade-show with medium=offline) are reported as 'Unassigned' and disappear from budget decisions. This tool predicts the channel instantly using Google's rules.
How to avoid Unassigned traffic
The most common cause is a non-standard medium (story, bio, influencer, offline) combined with a source GA4 doesn't recognise. The rule is simple: platform in utm_source, channel type in utm_medium, using mediums GA4 recognises (paid_social, cpc, social, email, referral, affiliate, sms): source=instagram and medium=paid_social for an Instagram ad, source=whatsapp and medium=social for a WhatsApp share. For QR codes on printed material, a recognised medium such as referral keeps the traffic out of Unassigned; create a custom channel group in GA4 if you want to track offline separately.
A UTM naming convention
GA4 treats UTM values as case-sensitive: 'Facebook' and 'facebook' are reported as two sources. Use one standard across the team: lowercase only, hyphens between words, ASCII instead of accented characters and a fixed list of sources. Add the period to campaign names (black-friday-2026) so you can compare year over year, and keep utm_content short and consistent for creative tests (video-a, video-b).
Google Ads: UTMs or auto-tagging?
With auto-tagging on, Google Ads appends a gclid parameter to ad clicks. According to Google's GA4 help, when gclid and manual UTMs are both present, source, medium and other traffic-classification dimensions use the auto-tagged values, while manual utm_term and utm_content still populate separate 'manual' dimensions. For Google Ads, keep auto-tagging on and link GA4 to your Ads account; use manual UTMs for Meta, LinkedIn, TikTok, email, influencers and SMS.
Where not to use UTMs
Don't add UTMs to internal links on your own site: reading new campaign data mid-visit overwrites the visitor's original source and breaks attribution. Don't expose UTM URLs to organic search either; canonical tags should point to the clean URL. If long UTM links look messy on social, use a link shortener — it keeps the parameters through the redirect.