The Google tag is a single piece of JavaScript, known in code as gtag.js and formerly called the global site tag, that sends information from your website to Google Ads, Google Analytics 4 and other Google products. One installation can serve several Google accounts, which Google calls destinations.
How the Google tag works
Each Google tag has an ID: one starting “AW-” for Google Ads, “G-” for GA4, or a combined “GT-” ID. The tag loads on every page, records page views and, through events, records actions such as a form submission or a purchase. Google Ads uses those events as conversions; GA4 uses them for reporting.
You can install it in two ways. Paste the snippet into the head of every page, often through a CMS plugin, or deploy it through Google Tag Manager, which gives you more control over when tags fire. Either works; doing both at once does not.
The Google tag screen inside Google Ads and GA4 holds settings that used to need code, such as which domains count as one site for cross-domain tracking, whether to collect user-provided data for enhanced conversions, and which URL parameters to ignore.
The tag also stores Google’s click identifier in a first-party cookie, so a later conversion can be credited to the ad click that started the visit. In Tag Manager setups, a conversion linker tag does that job.
Why it matters
Without a working Google tag, Google Ads cannot see your conversions, and automated bidding is left guessing. A broken or duplicated tag is one of the most common reasons I find for an account reporting twice the sales the business actually made, or none at all.
For a UK site there is a legal side. The tag sets and reads cookies, and under PECR, advertising cookies need the visitor’s consent first. The tag should therefore either wait until a UK visitor has made a choice on your cookie banner, or run in Consent Mode, where it starts with storage denied and changes behaviour only once consent is given. Consent Mode uses four settings: ad_storage, analytics_storage, ad_user_data and ad_personalization. Google requires them to be passed for UK and EEA traffic if you want to use features such as remarketing.
At the time of writing (October 2026), the Data (Use and Access) Act 2025 is relaxing the consent rule for some low-risk analytics cookies, but advertising cookies still need consent. Check the ICO’s current guidance before changing your setup.
Common mistakes
- Installing the tag twice, through a website plugin and through Tag Manager, so every conversion counts double.
- Firing the tag before the cookie banner has been answered.
- Consent Mode defaults set after the tag has already loaded, so the first page view ignores them.
- Old tag IDs from a previous agency still sending data to accounts you no longer control.
- Conversion events attached to a button click rather than a confirmed submission, so failed form sends are recorded as leads.
How to act on it
Open your site in Google Tag Assistant and note every Google tag ID it finds. Each should be one you recognise and own. Then accept and reject cookies on your own banner and check what fires in each case: with consent rejected, no advertising cookies should be set.
Submit a real test enquiry and confirm one conversion appears in Google Ads, not two. If anything looks wrong, fix the tag before changing bids or budgets, since every other decision depends on that data. I check this first whenever I take on an account through Google Ads management.