OnSumoTools

On-Page SEO Checker Methodology

What the tool measures, how it scores and where it stops.

What it does

  • Fetches one public page (raw HTML, first 1.5 MB, 12 s limit) or analyses HTML you paste. Pasted HTML is never saved or fetched.
  • Runs fixed rules. There is no AI scoring, so the same HTML gives the same result.
  • Also reads robots.txt, up to a few sitemap files, tests up to 10 links, and checks the canonical target.

How the score works

  • Every check is pass, info, warning or critical. The score starts at 100.
  • Each warning removes 5 points and each critical issue removes 10. Pass and info checks never change the score. The minimum is 0.
  • The score reflects HTML signals only. It is not a Google ranking, traffic or quality prediction.

Search snippet preview

  • Widths are estimated with Arial character widths: titles at 20px (limit about 600px) and descriptions at 14px (limit about 920px).
  • Google rewrites titles and descriptions and uses its own fonts, so treat the preview as a guide.

Structured data

  • JSON-LD blocks are parsed and checked against the required and recommended properties in Google's rich result documentation for common types (Product, FAQPage, BreadcrumbList, Article, Organization, LocalBusiness, Recipe, Event, JobPosting, VideoObject and more).
  • Missing required properties raise a warning. Missing recommended properties are informational. Invalid JSON-LD is ignored.

Speed (Core Web Vitals)

  • The optional speed check calls Google's PageSpeed Insights. Real-user LCP, INP and CLS come from the Chrome UX Report when Google has enough data for the page. Otherwise only lab results are shown.
  • Lab results are a single simulated mobile run and vary between runs.

Limits

  • Raw HTML only. Content added by JavaScript after load is not seen unless you paste the rendered HTML.
  • Pasted HTML cannot show HTTP headers such as X-Robots-Tag.
  • Some sites block automated checkers. Checks are limited to 20 per 10 minutes per visitor.
  • Before/after comparison uses your own browser storage and is not synced anywhere.