Migrate from Google Analytics 4 to Fathom: Step-by-Step Guide

When this migration makes sense

Fathom is the right destination if your top GA4 reports are “how many visits, what pages, where from” — and nothing else. Solo founders, small content sites, indie SaaS landing pages. If you find yourself wanting custom dimensions, audiences, or e-commerce schemas, Fathom is the wrong tool — look at Matomo or PostHog.

Pre-migration checklist

1. Confirm scope. List your top 10 GA4 reports. If more than two require custom events or dimensions, Fathom won’t replace them.
2. Inventory goals. Each becomes a Fathom “Event” with a fixed name. Fathom has 50 events per site limit.
3. Decide on EU-isolation mode. Fathom has an EU-only data routing option for compliance — slightly slower, but keeps data within EU borders.
4. Plan the cutover. Fathom doesn’t have a rich import — you start fresh. Archive GA4 BigQuery export for historical reference.

Step-by-step

1. Sign up + install (Day 1)

<br />
<script src="https://cdn.usefathom.com/script.js" data-site="ABCDEFGH" defer></script><br />

Replace ABCDEFGH with your site code from Fathom dashboard. EU-isolation mode adds data-eu="true".

2. Configure goals (Day 1-2)

In Fathom dashboard, Events → New Event. Each event gets a unique ID. Fire from your code:

<br />
fathom.trackGoal(‘SIGN_UP_GOAL_ID’, 0);  // 0 = no revenue<br />
fathom.trackGoal(‘PURCHASE_GOAL_ID’, 9900);  // 9900 = $99.00 in cents<br />

3. Map GA4 events to Fathom goals (Day 2-3)

Fathom doesn’t have GA4-style events with parameters. You map each GA4 event name → Fathom goal ID. Common mappings:

• GA4 sign_up → Fathom goal “Sign Up” (no revenue)
• GA4 purchase → Fathom goal “Purchase” (revenue in cents)
• GA4 add_to_cart → Fathom goal “Add to Cart” (revenue in cents)
• GA4 begin_checkout → Fathom goal “Begin Checkout”

Goals beyond ~20 become hard to manage in Fathom’s UI. Group aggressively.

4. Dual-tag (Days 3-14)

Run both for 14 days. Use the Parallel-Run Validator to compare daily totals.

5. Cutover (Day 15)

Remove GA4 snippet. Keep BigQuery export for one more billing cycle as a hedge.

Common gotchas

• Goal IDs are opaque — Fathom assigns random IDs. Document the GA4-event → Fathom-ID mapping in your team wiki.
• No custom properties — You can’t pass arbitrary metadata with goals. Revenue (cents) is the only parameter.
• SPA tracking — Fathom auto-detects pushState route changes since v3.4. Older versions need manual fathom.trackPageview().
• EU isolation perf — EU-routing adds ~30ms TTFB on the script. Acceptable, document for stakeholders.