Font Size Signals Test
The Font Size Test scans every block of visible text on a webpage at the mobile viewport (375px) and reports whether the rendered font size is large enough to read without zooming.
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The Font Size Test scans every block of visible text on a webpage at the mobile viewport (375px) and reports whether the rendered font size is large enough to read without zooming. Google's mobile-usability spec calls for at least 16px equivalent for body text, and Lighthouse fails any page where more than 60% of visible text falls below that threshold. Pages with tiny mobile text bounce hard and trigger explicit usability warnings in Search Console.
What This Tool Checks
- Body font size at the 375px viewport
- Per-element font-size distribution
- Percentage of visible text below 16px
- Use of relative units (em, rem) vs. fixed px
- Anti-aliasing and rendering smoothness
- Lighthouse "Document does not use legible font sizes" check
Why It Matters for SEO
Tiny text on mobile drives bounce. Users zoom, get frustrated and leave — destroying both UX and conversion metrics. Google measures this directly via Lighthouse and the mobile-friendly test, and pages that fail are noted as "Text too small to read" in Search Console's mobile usability report. Restoring legible font sizes typically improves time-on-page, reduces bounce by 10-30% and removes the usability warning.
How to Fix It
Set the root font-size to 16px (or browser default) and size body text in 1rem. Use relative units throughout. Audit small captions and footnotes — never go below 14px on mobile. Re-test in Lighthouse to confirm the legible-font-sizes check passes.
How It Works
We render the page at a 375 x 667 mobile viewport in headless Chrome, walk every visible text node, capture its computed font-size, and aggregate the distribution. Pages that fail Lighthouse's 60% threshold are flagged with the specific elements responsible.
Common Mistakes to Avoid
- Body text set in fixed px below 16
- Font-size declared in em without a sensible root font-size
- Mobile-specific media query overriding desktop sizes downward
- Captions or footnotes at 12px hidden in important content
- Pixel-perfect designs from desktop pushed to mobile without scaling up
Quick Checklist
- Body text at least 16px on mobile
- No important text below 14px
- Relative units (rem) used throughout
- Lighthouse legible-font-sizes audit passes
- No "Text too small to read" warnings in Search Console
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16px is Google's mobile-usability baseline for body text. Smaller sizes (down to 12px) are acceptable for legal copy and footnotes but should never be the dominant text size.
Google's mobile-friendly test scores legibility directly. Pages that fail show up in Search Console under "Mobile Usability" and are demoted in mobile rankings until fixed.
Yes. Rem respects the user's browser default font-size (and operating system accessibility settings). Pixel sizes ignore both, breaking accessibility for users who set larger text system-wide.
Yes — overly large body text wastes screen space and forces excessive scrolling. Aim for 16-18px body and use scale (1.125-1.25) for headings.
Use Chrome DevTools mobile emulation (Ctrl+Shift+M), inspect any text element, and read the computed font-size. Or run Lighthouse to get a sitewide legibility score with specific failing elements listed.