Not every page a search engine crawls ends up in the index. Pages can be excluded because they carry a noindex directive, are duplicates consolidated under a canonical, are blocked by robots.txt, or were deemed thin or unhelpful.
A healthy site typically has an index coverage ratio above 70–80%. A low ratio — say, 30% — means a significant portion of the site is invisible to searchers despite being crawlable.
Causes of a low ratio:
- Noindex tags applied too broadly (e.g., accidentally on template pages)
- Duplicate content — paginated pages, filtered URLs, or session-parameter variants that should be canonicalised
- Thin pages that Google excludes on quality grounds
- robots.txt blocking pages that should be indexed
What SearchVitals checks: The Crawl & Index audit compares pages discovered during crawling against Search Console indexing data to compute the coverage ratio, flagging low ratios as a warning or critical finding.