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. 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