301 Redirect or Canonical for Merging Two Sites: What's the SEO Difference?

What is the technical difference between a 301 redirect and a canonical?

The 301 redirect is a server instruction that automatically sends the visitor and bots from point A to point B. It signals a permanent move. The original URL disappears from the index in favor of the new one.

The canonical, on the other hand, is an HTML tag that says: "This page exists, but consider this other URL as the reference instead." Both URLs remain accessible. Google chooses which one to index, but the old one can still receive direct traffic.

Why does Google make this distinction for site mergers?

Because business objectives differ. A 301 redirect involves a complete abandonment of the old site: you no longer want it to exist in the eyes of Google or users. It's a final merger.

The canonical, conversely, acknowledges that the old site may have a reason to exist outside of SEO: a specific geographic domain (.fr vs .com), contractual obligations, or a particular technical version. You tell Google: "Focus your crawl and ranking on the new, but do not remove the old from your servers."

In what concrete scenarios does this distinction apply?

An e-commerce site migrating from anciensite.com to nouveausite.com without retaining the old domain will use 301 redirects. The goal is to transfer 100% of the authority, clean the index, and remove the old property.

A company merging two brands but needing to maintain the old domain for B2B clients or existing contracts will use the canonical. The old site remains functional, but Google no longer ranks it. It’s a tactical choice when complete removal is not possible.

• 301 Redirect: complete merger, authority transfer, old site deindexed and inaccessible
• Canonical: old site remains active, SEO signal redirected, temporary or permanent dual presence
• Crawl impact: 301 immediately frees up budget, canonical maintains two crawlable URLs
• Risk of dilution: canonical can create confusion if misconfigured, 301 is clearer
• Migration timing: 301 transfers authority within weeks, canonical may take longer depending on crawl frequency

Is this recommendation consistent with real-world practices?

Yes, and it's actually one of the few statements from Mueller that aligns perfectly with what we observe in real migrations. 301 redirects are the industry standard for any site merger where the old domain no longer has a reason to exist. Authority transfer typically occurs within 3 to 6 weeks if the migration is clean.

The canonical, however, is underutilized in this specific context. Many SEOs use it for internal duplicates, but few consider it as a solution for controlled coexistence between two domains. Yet, in migrations where the client refuses to cut the old site for legal or commercial reasons, it is the only viable option. [To be verified]: Google does not specify how long this coexistence can last without a gradual loss of authority.

What nuances should be added to this distinction?

The canonical is not a guarantee that Google will respect your choice. If the old site receives more fresh backlinks, better user engagement, or is technically better optimized, Google may ignore the canonical and continue indexing the old version. It's a signal, not an order.

Additionally, maintaining two sites active with canonical doubles the operational burden: two domains to monitor, two SSL certificates, two technical infrastructures. If the old site goes down or accumulates 500 errors, it can disrupt the signal to the new one. The 301 cuts this complexity cleanly.

In what cases does this rule not apply?

If you're migrating a site with international subdomains (fr.site.com, de.site.com), neither 301 nor canonical is the right approach. You need to use hreflang tags to indicate language versions. Canonical would break the multilingual structure.

Another case: migrations in multiple phases. If you're merging gradually (category by category), mixing 301 and canonical can create canonicalization loops. It's better to break the migration into coherent blocks and apply a single method per block. Beware: a canonical from domain A to domain B, which itself redirects in 301 to domain C, may be ignored by Google.

What should you do to choose between 301 and canonical?

First, ask yourself a simple question: Does the old site need to remain accessible after the merger? If the answer is no, then a 301 is necessary. If the answer is yes, dig deeper: why does it need to stay accessible? For SEO reasons? Then the canonical is unnecessary; it’s better to keep both sites independent. For commercial or legal reasons? Then the canonical becomes relevant.

Next, assess the technical complexity. A 301 migration requires rigorous URL mapping, but once launched, it is irreversible and clear. The canonical requires ongoing maintenance: monitoring that Google respects the signal, ensuring no new backlinks come to the old site, and checking that both domains remain performant. If you don't have the resources to monitor two properties, the 301 is safer.

What mistakes should be avoided in implementation?

The first classic mistake: putting a canonical from site A to site B but not disabling the old sitemap. You send contradictory signals. If you use the canonical, remove the old site from Search Console, delete or mark the sitemap as obsolete, and stop any active promotion of the old domain.

The second mistake: thinking that the canonical exempts you from migration tracking. No. You must track traffic distribution between the old and new sites, monitor positions on both domains, and verify in Search Console that Google consolidates crawl on the new one. If after 3 months you still see 50% of organic traffic on the old site, it means the canonical is not being respected.

How can you verify that the chosen strategy is working?

For a 301 migration, monitor in Search Console the gradual drop in indexed pages from the old domain and the symmetric rise on the new one. Use a position tracking tool to check that the new domain URLs are picking up the rankings of the old one. A good indicator: 80% of the old organic traffic transferred within 6 to 8 weeks.

For a canonical setup, regularly inspect the URL with Google's inspection tool. It should indicate "User-defined canonical URL" and display the new domain as the reference. If Google chooses a different canonical URL than the one you've declared, it’s a warning signal: either the old site is stronger technically, or your signals are inconsistent.

• Decide upfront whether the old site should disappear (301) or remain active (canonical)
• Map all old URLs to their new equivalents before any action
• If 301: implement the redirects at the server level (Apache, Nginx, CDN), not in JavaScript
• If canonical: place the tag in the <head> of each page on the old site, pointing to the new domain
• Remove the old sitemap from Search Console if you're using the canonical
• Track the evolution of indexed pages and organic traffic on both domains for at least 3 months