60-Second Summary
This guide explains how inconsistent capitalization in UTM parameters fragments campaign data in GA4 and shows how to merge existing splits and prevent them going forward. It covers which fixes work (custom channel groups, report-level LOWER() transforms, BigQuery) and why GA4 data filters won't rewrite past values.
Key takeaways: Capitalisation in UTM values (e.g., LinkedIn vs linkedin) creates separate source/medium/campaign rows in GA4 and can undercount or break campaign comparisons; default channel groups are case-insensitive but the split appears at the source/medium/campaign level. GA4 data filters cannot rewrite or correct already-collected values.
Standout strategies & tactics: Use a custom channel group to merge variants retroactively; in reports use LOWER() (Data Studio) or lowercase in BigQuery queries to normalise displayed values; build links from a controlled builder and run a UTM checker or validate in GA4 Realtime before publishing.
Real-world lessons & frameworks: Prevent issues with a strict lowercase UTM naming convention, provide approved values to agencies and external teams, prefer builders over manual typing, and enforce link checks—prevention is far simpler than retroactive fixes.
*This summary was created with AI assistance, using our original content.
This guide shows what that does to your reports, how to merge data that is already split and how to stop it happening again. For the basics of what each tag records, see our guide to UTM parameters.
What case sensitivity does to your GA4 reports
Take a LinkedIn campaign promoting a webinar. One person tags the ad link with utm_source=LinkedIn. Another tags a second ad with utm_source=linkedin. In GA4, the campaign now appears as two rows:
Each row holds part of the campaign. Anyone who reads only the top row underestimates the campaign by 40%. A filter for linkedin misses the LinkedIn row entirely, and a comparison between this month and last month falls apart if the capitals changed in between.
The problem does not stay in one report. Every exploration, dashboard and Data Studio chart built on source, medium or campaign inherits the split, and the more people who build links, the more versions you collect. Our guide to the GA4 UTM report shows where these rows appear.
Are UTM parameter names case sensitive too?
Google's help pages only ever show the parameter names in lowercase, and they do not say whether capitalised versions such as UTM_SOURCE are read.
So write utm_source, utm_medium and utm_campaign in lowercase every time. There is no reason to capitalise them and a real chance it stops the tags being read.
The same logic applies to web addresses in general. Everything after the question mark in a link is read exactly as typed, so capitals count there even when they do not count in the domain name.
Does case affect GA4 channel groups?
No. Google's default channel group help page says the channel definitions are not case sensitive. Visits tagged LinkedIn / paid_social and linkedin / paid_social both land in the Paid Social channel.
So a channel-level report, such as Traffic acquisition grouped by Session default channel group, still shows the right total for Paid Social. The split happens one level down, in the source, medium and campaign dimensions, which is where most campaign analysis takes place. That is why case problems often go unnoticed until someone tries to compare individual campaigns.
How to fix split UTM data you already have
GA4 filters cannot do it
GA4's data filters can only include or exclude certain traffic, such as developer traffic, internal traffic or traffic from specific hostnames. They cannot rewrite or lowercase values, and according to Google's data filters help page, they do not change data that has already been collected. If you set up a filter and nothing changed, this is why.
Group the values with a custom channel group
A custom channel group lets you define your own channels with rules that match several values, so LinkedIn and linkedin can sit in one channel of your choosing. Custom channel groups apply to past data as well as new data, so they also tidy up reports for campaigns that have already run. A standard GA4 property can have two, so plan them before you create one.
Lowercase the values in Data Studio
In Data Studio, create a calculated field that wraps the source, medium or campaign field in the LOWER() function, such as LOWER(Session source), and build your charts on that field instead. Google's LOWER() documentation covers the syntax. This fixes the report, not the data in GA4, but for most teams the report is what matters. Teams that export GA4 data to BigQuery can apply the same function in their queries.
How to stop it happening again
Fixes after the fact are workarounds. The lasting answer is to stop mixed capitals reaching GA4 in the first place.
Make lowercase a written rule in your UTM naming convention, with no exceptions for brand names or acronyms. linkedin, google and webinar-b2b-analytics-2026-11 are all lowercase, and nobody has to remember which platforms are capitalised.
Build links with a builder instead of typing them, so the values come from a list rather than someone's memory. And check every link before it goes live: check your UTM links with a checker, or click each one and confirm the values in GA4 Realtime.
If agencies or other teams build links for you, send them the convention and the approved values before their first campaign. Most mixed capitals come from someone outside the core team who was never told the rule.
Our UTM builder lowercases every value as you type, and the UTM checker flags capital letters in a pasted link and gives you a corrected version to copy.
Frequently asked questions
Are UTM parameters case sensitive in GA4?
Yes. GA4 treats UTM values as case sensitive, so utm_source=LinkedIn and utm_source=linkedin appear as two separate sources in your reports. Google's own documentation confirms this and recommends using lowercase for every value. Default channel groups are not case sensitive, so the split shows up in source, medium and campaign reports.
Can I fix UTM case problems in past GA4 data?
Not inside the data itself. GA4 cannot lowercase values after they are collected, and data filters do not change past data. A custom channel group can group the variants together and applies to past data, and Data Studio's LOWER() function can merge them in a report.
Should UTM parameters be lowercase?
Yes. Write every UTM value in lowercase, including platform names, campaign names and content labels, and write the parameter names in lowercase too. It removes a whole category of reporting errors, and it matches Google's own recommendation. Put the rule in your naming convention so everyone on the team follows it.
Does capitalisation in UTM parameters affect SEO?
No. UTM parameters on links from ads, emails and other websites do not affect how your pages rank, whatever their capitalisation. What capitals do affect is your analytics, by splitting one source or campaign into several rows. Keep a self-referencing canonical tag on each page and keep UTM values lowercase.
Build your next links with the free UTM builder, so the values are lowercase before they reach GA4.
