Conversion tracking: measuring what your ads lead to

What is conversion tracking?

A conversion is an action that has value for your business, such as a purchase, a sign-up or a call. Conversion tracking records these actions and attributes them to the ad the person interacted with beforehand. This shows you which advertising spend leads to results.

This data feeds metrics such as the conversion rate, CPA and ROAS. Automated bidding relies on it too, as our article on Smart Bidding explains. This article covers Google Ads. Measurement for Meta ads is explained in our entry on the Meta Pixel.

A hypothetical example: a campaign costs €1,000 and, according to tracking, brings 25 purchases with €3,000 in revenue. That works out at €40 per purchase and a ROAS of 300%. Without conversion tracking, you would only see clicks and costs for the same campaign.

How conversion tracking works

For conversions on a website, measurement works as follows, according to Google Ads Help:

1. Conversion action: you define in Google Ads what gets counted.
2. Tag: a Google tag or code snippet is added to the website.
3. Click: after a click on an ad, a temporary cookie is stored on the device.
4. Action: when the person completes the action, the system recognises the cookie and records the conversion.

If consent is missing or the browser blocks the cookie, gaps appear in the data. Read more in our article on cookies and in our comparison of client-side and server-side tracking.

What you can measure

In Google Ads, every conversion action is primary or secondary. Primary actions appear in the “Conversions” column and feed into bidding if they belong to a goal used for bidding. Secondary actions are for observation only and appear in “All conversions”. Conversions imported from Analytics are secondary by default. Our article on GA4 explains how to create key events.

Setting up conversion tracking

For a website using Google Tag Manager, these are the steps:

1. Define the goal: write down which action counts and what it is worth.
2. Create the action: create a conversion action in Google Ads under Goals and Conversions. The conversion ID and conversion label are in the tag setup area.
3. Create the tags: in Tag Manager you need the Google tag, a Conversion Linker tag and a Google Ads Conversion Tracking tag with the ID and label.
4. Pass the value: conversion value, transaction ID and currency are optional. If the value is empty, Google Ads uses the default value set in the conversion action. Shops and forms usually provide these details through the data layer.
5. Choose a trigger: the trigger determines when the tag fires, for example when a form is submitted.
6. Test and publish: use preview mode to check that the tag fires once and passes the correct value.

Sample code without Tag Manager

If only the Google tag is installed, this event snippet reports a purchase. It goes on the order confirmation page. The conversion ID and label come from your conversion action, the other values from your shop:

<script>
gtag('event', 'conversion', {
  'send_to': 'AW-123456789/AbC-D_efG-h12_34-567',
  'value': 49.90,
  'currency': 'EUR',
  'transaction_id': 'B-10042'
});
</script>

The value is a number with a decimal point, and the currency is a three-letter code. Your server must generate a new transaction ID for each order. Google does not count a second conversion with the same ID. An ID hard-coded on the page therefore leads to missing conversions.

Checking your setup: the conversion action status

You will find the status under Goals, then Conversions, then Summary, in the “Status” column:

Counting method and conversion window

Two settings of each conversion action change your figures. The counting method sets how many conversions count per ad click. Google recommends “Every” for purchases and “One” for leads, because usually only the first enquiry after a click adds value.

The conversion window sets how long after the ad interaction an action still counts:

A longer window makes sense for long decisions, such as expensive purchases. Changes only apply to future conversions.

Enhanced conversions

Enhanced conversions add first-party customer data to your conversion data, such as an email address, name, address or phone number. According to Google, the data is hashed with the SHA-256 algorithm before it is sent. Google matches the hashed values with Google accounts and can attribute conversions more accurately as a result. This is intended to improve measurement and give bidding more data.

Google has so far distinguished two variants:

• For web: sales and events on the website, with user data from the order form, for example.
• For leads: deals closed outside the website that started with an enquiry, together with imported offline conversions.

According to Google Ads Help, Google Ads has accepted user-provided data from website tags, Data Manager and API connections in parallel since April 2026. Starting in June 2026, Google is combining both variants into one feature with an on/off switch, so that advertisers no longer choose a method. Since 15 June 2026, Google has been moving offline imports and lead uploads made through the Google Ads API to the Data Manager API and blocking them in the Google Ads API. File imports are still listed in Google Help as a separate, older route. Google migrates existing users who have already accepted its customer data terms automatically, and the feature can be switched off for individual conversion actions. Check the status in your account, as Google often rolls out changes gradually.

If you hash data yourself, it must be normalised first. Google requires lower case, no spaces at the beginning or end and phone numbers in E.164 format, among other things.

Google Ads and GA4: why the figures differ

The two systems count the same purchases differently. According to Google Ads Help, these are the most common reasons:

A difference alone therefore does not indicate an error. Check a single action over the same period and with the same filters.

Consent: our assessment

This section is an assessment and does not constitute legal advice. Conversion tags work with cookies. Under section 25 of the German TDDDG, this generally requires consent. In the European Economic Area, Google also requires consent to be passed on to Google tags, usually via Consent Mode v2.

Enhanced conversions add customer data. In our view, hashing does not make this data anonymous, because Google matches the values with accounts. Include the process in your consent wording and privacy policy, and check it with your data protection officer. Our consent management service covers the technical side.

Common conversion tracking errors

• Double counting: the same purchase runs through a Google Ads tag and an imported GA4 event, both as primary actions. One source is enough for bidding.
• Reloaded thank-you page: the tag fires again. A transaction ID helps to identify duplicate purchases.
• Tag fires too early: it fires when someone clicks “Submit”, even if the form then shows an error. The trigger should be tied to successful submission.
• Missing values: without a value passed in, Google Ads counts every conversion at the same default value, which makes metrics such as ROAS unusable.
• Too many primary actions: if a newsletter click counts like a purchase, automated bidding also aims for it.
• Frequent changes to goals: larger changes to conversion goals send the bid strategy back into learning. Google shows the “Learning” status for up to seven days and recommends judging performance afterwards over two conversion cycles or two weeks, whichever is longer.
• Comparing Google Ads and GA4: the figures differ, partly because of different attribution models. Google Ads conversions also do not appear in GA4 standard reports.