# Canonical URL signals

> Redirects, rel="canonical" and sitemap inclusion all tell Google which duplicate URL you prefer. Google treats them as signals and picks the canonical itself. Redirects and rel="canonical" are both strong signals, with redirects listed first; sitemaps are weak.

- URL: https://geo.casa/crawling-indexing/canonical-signals/
- Reference · Officially documented
- Reviewed: 2026-10-24

## Methods

| Method | Strength | Notes |
|---|---|---|
| 301 / 308 redirect | Strong | Use when the duplicate should no longer be visited. |
| `<link rel="canonical">` in HTML | Strong | Absolute URL in `<head>`. |
| `Link: <…>; rel="canonical"` header | Strong | Works for non-HTML files such as PDFs. |
| Inclusion in a sitemap | Weak | Lists preferred URLs; easy to scale but weak. |

> **Officially documented:**
> Google ranks these methods by strength. Redirects and `rel="canonical"` are strong signals, and sitemap inclusion is weak. The signals can stack, so combining them increases the chance your preferred URL is chosen.
>
> Sources: [Google Search Central: How to specify a canonical URL with rel="canonical" and other methods](https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls)

## Rules of thumb

- Each indexable page should declare a self-referencing canonical.
- Don't point a canonical at a URL that is redirected, `noindex`, or blocked by robots.txt.
- Don't use `robots.txt` for canonicalization ([robots.txt](https://geo.casa/crawling-indexing/robots-txt/) controls crawling only).
- For language versions, use [hreflang](https://geo.casa/crawling-indexing/hreflang/), not a canonical pointing across languages. Each translation canonicalizes to itself.

## Sources

- [Google Search Central: How to specify a canonical URL with rel="canonical" and other methods](https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls)