URL Canonicalization Test

What is it?

When the same content is reachable at multiple URL variants, apex versus www, HTTP versus HTTPS, with or without a trailing slash, search engines treat each one as a separate page and split ranking signals across them. Backlinks, internal links, and click-through history that should consolidate behind a single canonical URL end up diluted, weakening rankings on every variant. This test verifies that the major URL variants of your site resolve consistently to one canonical version through permanent 301 redirects. The Google Search Central canonicalization guide documents the full set of factors search engines use to choose a canonical URL.

Why canonicalization matters

Search engines treat each unique URL as a separate page, even when the content is identical. If both https://example.com and https://www.example.com serve the same homepage, that homepage's accumulated ranking signals (backlinks, internal links, click-through history) get split across two URLs that should be one. The result is weaker rankings on both variants than the consolidated single URL would achieve. Canonicalization fixes this by funneling every variant to a single chosen URL via permanent 301 redirects.

The same problem applies to other URL variants: HTTP and HTTPS, trailing slash and no trailing slash, uppercase and lowercase paths, and index file variants such as / versus /index.html. Each pair where both versions resolve to content represents a potential signal split. The remedy is consistent: pick one canonical form and 301-redirect every other variant to it permanently.

Common canonicalization issues

Both apex and www serve content: pick one and 301-redirect the other.
HTTP and HTTPS both serve content: redirect HTTP to HTTPS and add HSTS so browsers stop trying HTTP at all.
Trailing slash inconsistency: pick one form and redirect the other.
Uppercase or mixed-case URLs coexisting with lowercase canonicals.
Index page accessible at multiple paths such as /, /index.html, and /index.php.

This test verifies that the major URL variants of your site resolve consistently to a single canonical version. The fix guide below covers configuring 301 redirects in the major web servers, CDN platforms, and hosting providers, plus the role of the <link rel="canonical"> tag as a backup signal.

Pass rate:

Top 100 websites: %
This value indicates the percent of top 100 most visited websites in the US that pass this test (in the past 12 months).
All websites: 71%
This value indicates the percent of all websites analyzed in SEO Site Checkup (500,000+) in the past 12 months.

Pass rates of Top 100 US websites
2021	92%
2022	96%
2023	97%
2024	93%

100

How do I fix it?

This test fails when different URL variants of the site (apex versus www, HTTP versus HTTPS, with or without trailing slash) do not all resolve consistently to a single canonical version. When the same content is reachable at multiple URLs, search engines may split ranking signals across the variants. Fixing this issue means picking one canonical form for the site and 301-redirecting every other variant to it. See the Google Search Central canonicalization guide for the full specification.

Where to make the change

Server configuration: add 301 redirects in your web server config (Nginx, Apache, IIS) or via _redirects on Netlify, vercel.json on Vercel, or Cloudflare Page Rules.
CDN: most CDNs offer redirect rules that fire before the origin is hit, which is the cheapest way to enforce canonicalization.
Application code: as a backstop, applications can detect the wrong host or scheme and emit a 301 redirect to the canonical URL.

Common causes and how to resolve them

Both apex and www serve the same content: pick one (most sites prefer apex today) and 301-redirect the other to it permanently.
HTTP and HTTPS both serve content: redirect all HTTP requests to HTTPS, then add HSTS so browsers stop trying HTTP at all.
Trailing slash inconsistency: pick one form (with or without) and redirect the other.
Uppercase or mixed-case URLs: redirect uppercase paths to their lowercase canonical form.
Index page accessible at multiple paths: for example /index.html, /index.php, and / all serving the same content. Redirect everything to /.

Best practices

Use 301 (permanent) redirects: they pass the strongest ranking signal and are cached aggressively by browsers.
Pair with <link rel="canonical">: the canonical tag is a backup signal for cases the redirect cannot cover (parameterized URLs, syndicated content).
Avoid chains: redirect directly to the final URL in one hop.
Update internal links to canonical URLs: linking to non-canonical variants triggers unnecessary redirects on every visit.