Custom parameters are named values you create in the URL options of a Google Ads campaign, ad group, ad or keyword, which Google then inserts into your tracking links when someone clicks. They let you pass your own information, such as a promotion code or a landing page version, to your website and analytics without editing every final URL.
How custom parameters work
A custom parameter has a name, which you write inside curly brackets with a leading underscore, and a value. For example, you might set {_promo} to “autumnsale” at campaign level. You then reference it in a tracking template or final URL suffix, such as utm_content={_promo}. When an ad is clicked, Google swaps the placeholder for the value, so the visitor lands on a URL ending utm_content=autumnsale.
They work alongside Google’s own ValueTrack parameters, which fill in values Google knows, such as {keyword} or {device}. The difference is that custom parameters carry values only you know: an internal product line code, a creative test name, a sales region. Google sets limits on how many custom parameters each level can hold and on the length of names and values, so keep them short.
Parameters set at a lower level override the same name at a higher level. If {_promo} is “autumnsale” on the campaign but “clearance” on one ad group, clicks from that ad group carry “clearance”. That inheritance is what makes them useful: one tracking template at account level can produce different values across thousands of ads.
Why it matters
Most small UK accounts never need custom parameters, because auto-tagging and a sensible set of UTM tags already tell Google Analytics where a visit came from. They become useful when the business needs to pass details the standard tags do not cover. A London estate agent running separate ad groups for lettings and sales might use {_dept} to route form enquiries to the right team in the CRM. A retailer testing two landing page designs might pass {_lp} so the site serves the right version and reports separate them.
Changing a parameter value also does not send ads back for review in the way that editing the final URL can, so you can switch a promotion label across a campaign quickly.
Common mistakes
- Referencing a parameter in a template but never defining a value, so the URL arrives with an empty or literal placeholder.
- Writing the placeholder without its leading underscore, as {promo} instead of {_promo}, so Google never substitutes the value.
- Using spaces or unusual characters in values, which then appear encoded and messy in analytics.
- Defining the same name at several levels and losing track of which value wins.
- Building an elaborate parameter scheme nobody reads, while basic conversion tracking is still broken.
How to act on it
First decide whether you need them. If your questions can be answered by campaign, ad group, keyword and device, auto-tagging and UTMs already do the job. If you need a value Google cannot know, write down the name, the allowed values and where they are set.
Define the parameter at the highest level that makes sense, reference it in your account-level final URL suffix or tracking template, then click a live ad or use the URL test button to confirm the value arrives on the landing page. Check that your analytics or CRM actually stores it. Keeping URL tracking tidy is part of the work in monthly PPC management.