GA4 for E-commerce: How to Measure Real Revenue from Google Ads

The most common sentence we hear from e-commerce owners is not „I don't know how to launch a campaign" — it's „I don't know whether the campaign actually pays off". Google Ads shows one number of conversions, GA4 shows another, and at the end of the month the bank statement shows a third. The problem is almost never the campaign. The problem is measurement.

Google Analytics 4 (GA4) has been the only version since July 2023 — Universal Analytics is shut down. Yet most stores still use it as if it were UA: they look at sessions and bounce rate, not the only thing that matters — revenue by traffic source. This is a practical guide to configuring GA4 so it answers the question „how much does Google Ads really bring me".

01 — Why the Old Way of Measuring No Longer Works

UA was built on sessions. GA4 is built on events. Every interaction — product view, add to cart, purchase — is an event with its own parameters. For e-commerce that means you must send structured ecommerce events, not just pageviews. Without them GA4 sees traffic, but it doesn't see the money.

GA4 without e-commerce events is like a register that counts how many people enter the store, but not how much they spent. Traffic without revenue is vanity, not a metric.

02 — Four Events You Must Send

Google's recommended e-commerce schema has more events, but these four cover 90% of decisions:

Event	When it fires	Why it matters
view_item	Opening a product page	Measures per-product interest, feeds remarketing
add_to_cart	Adding to cart	Main intent signal; key for abandoned carts
begin_checkout	Starting checkout	Shows where in the funnel you lose buyers
purchase	Successful purchase	The only event with real revenue (value + items)

The most important is purchase because it carries value, currency and the items array. Without an accurate value and currency, GA4 and Google Ads cannot calculate ROAS.

// dataLayer push on the thank-you page
dataLayer.push({
  event: "purchase",
  ecommerce: {
    transaction_id: "ORD-10245",
    value: 129.90,
    currency: "EUR",
    items: [
      { item_id: "SKU-1024", item_name: "Wool Coat Camel M",
        price: 129.90, quantity: 1, item_brand: "MAISON" }
    ]
  }
});

03 — Linking GA4 and Google Ads

This is the step that's missing most often. A GA4 conversion means nothing for campaign optimization until you import it into Google Ads as a conversion source:

1. In GA4: Admin → Product links → Google Ads links — connect the Ads account.
2. Mark purchase as a key event (the former „conversion").
3. In Google Ads: Goals → Conversions → Import → GA4 and import the key event.
4. Disable duplicate conversions — if you have both a Google Ads tag and a GA4 import for the same purchase, you'll double-count and ROAS will be falsely high.

After this, Smart Bidding (Maximize conversion value / Target ROAS) learns from your actual revenue, not from click counts.

04 — Most Common Mistakes We See in Audits

• Hardcoded value. Every purchase sends the same value (e.g. 1) because the developer never wired the real cart total. ROAS becomes meaningless.
• Wrong currency or none. Without currency, GA4 assumes the account currency — often USD — so revenue is shown 100× wrong.
• Double counting. A Google Ads conversion tag and a GA4 import firing together without deduplication.
• Cross-domain not configured. If checkout runs on a subdomain or external payment gateway, without cross-domain measurement the referral becomes „payment-provider" and Google Ads loses attribution.
• Self-referral from your own site due to redirects — it breaks the traffic source.

05 — What to Do This Week

Open GA4 → Reports → Monetization → Ecommerce purchases. If it's empty or the numbers don't match your register — measurement is broken and every budget decision is made blind. Fix order by impact: purchase event with correct value/currency → Ads import → deduplication → cross-domain.

It's the same principle as with the feed: Google makes decisions based on the data you send it. A bad feed = weaker visibility. Bad measurement = budget spent without control.

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Follow the blog for the next part: Google Tag Manager setup and how to measure ROAS by channel.