Conversion tracking is the single most important technical setup in any Google Ads account. Without it, you are flying blind. With bad tracking, you are flying in the wrong direction. This guide walks through the correct way to connect GTM, GA4, and Google Ads so that every conversion is counted once, accurately, and attributed to the right campaign.

Why GTM is the right tool for this

Google Tag Manager lets you deploy and update tracking tags without touching your site's code. This matters because conversion tracking setups change: you add new conversion actions, you update your privacy consent configuration, you switch from Universal Analytics to GA4. Doing all of this through GTM means your developer does not need to be involved every time. It also gives you a single place to audit all the tags firing on your site.

Configuring the GA4 purchase event

For e-commerce, the GA4 purchase event should fire on the order confirmation page with the transaction ID, revenue, and item details passed as event parameters. The transaction ID is critical: it prevents the same purchase from being counted twice if a user refreshes the confirmation page. In GTM, use a Custom HTML tag or the GA4 Event tag with the ecommerce data layer variable to pass these parameters.

Importing GA4 conversions into Google Ads

Once your GA4 purchase or lead event is firing correctly, link your GA4 property to your Google Ads account in the GA4 admin panel. Then go to Google Ads, open Conversions, and import the GA4 event as a conversion action. Set the count to 'One' for lead forms and 'Every' for purchases. Do not create a separate Google Ads conversion tag for the same event. One source of truth only.

Consent Mode v2 configuration

From 2024, Google requires Consent Mode v2 for advertisers using Google's audience and measurement features in the EU and increasingly in other markets. In Japan, APPI compliance is moving in a similar direction. Implement Consent Mode via GTM by adding a consent initialization trigger that fires before all other tags, and configure the default consent state based on your privacy policy. This ensures that conversion modeling works correctly for users who decline cookies.

Auditing your setup

After setup, use GTM's Preview mode to verify that the conversion tag fires exactly once on the confirmation page. Then use the Google Ads Tag Diagnostics tool to check for any firing errors. Finally, run a test transaction and verify that it appears in both GA4's real-time report and in Google Ads' conversion column within 24 hours. If all three checks pass, your tracking is clean.

Clean conversion tracking is not a one-time setup. It needs to be audited whenever you change your site, update your CMP, or add new conversion actions. Our conversion tracking audit covers all of this for ¥60,000.