Canonical Tag Checker: Fix duplicate content.

What a canonical tag actually does

Canonical tags tell Google which URL is the “real” version of a page when several URLs render similar or identical content. They’re the single most-misconfigured element in technical SEO — a wrong canonical can quietly de-rank an entire site by telling Google to index a different page, or no page at all.

A correct canonical points the page at itself with an absolute, HTTPS, www-or-bare- consistent URL. Anything else is a footgun: pointing at an older version, a different protocol, the homepage, a 404, or — most commonly — a URL that uses the opposite trailing- slash convention from the rest of your site. Each of these tells Google to consolidate ranking signal somewhere other than where you intended.

What our canonical checker catches

• Self-reference failures — pages that don’t list themselves as canonical, the most common cause of duplicate content penalties.
• Canonical chains — A canonical→B canonical→C; Google follows the first link only and may stop following at the wrong destination.
• Protocol mismatches — HTTP page with an HTTPS canonical (or vice versa); the wrong version is what gets indexed.
• Host mismatches — bare-host page declaring a www canonical, or vice versa, when the rest of the site is consistent. Splits ranking equity in half.
• Trailing slash inconsistency — page at /about with canonical /about/, or vice versa. Tiny detail, real ranking impact.
• Cross-page canonicals — a product variant pointing canonical to the parent product. Sometimes correct, sometimes a bug; we flag for human review.
• Multiple canonical tags — having two or three rel=canonical tags on the same page; Google treats this as if you had none.
• Canonical to a 404 — pointing at a URL that returns 404 or 410; Google treats the page as orphaned.

Why most sites have at least one canonical bug

Canonical tags are usually rendered by a CMS template, which means a single template bug affects thousands of pages at once. WordPress sites typically inherit canonicals from Yoast/Rank Math; misconfigured plugins generate canonicals to the wrong protocol after a TLS migration. Headless sites built in Next.js or Astro often hardcode the canonical domain, which breaks when the site moves to a new domain or adds a staging environment. E-commerce platforms (Shopify, WooCommerce) use parametrised URLs (?variant=X) that need their own canonical handling, and the defaults are often wrong out of the box.

Our checker scans every URL in your sitemap, fetches the rendered HTML, parses every canonical tag, and compares against the URL it was served from. The output is a ranked fix list — by how much SEO impact each fix is likely to deliver — so you can start with the biggest wins. Read our deeper coverage on the canonical tags learn page for the full taxonomy of canonical mistakes.