Font Size Accessibility Checker

Legibility depends less on nominal type size than on how large the text is on the retina, which is why this checker converts pixels into a visual angle. A pixel at the CSS reference density of 96 DPI is about 0.2646mm, so the angle subtended is the arctangent of character height divided by viewing distance. At a desktop distance of 60cm, 16px text spans roughly 24 arcminutes; at 3 metres it spans about 5 arcminutes and is unreadable for most people. Around 12 arcminutes is used here as a floor for sustained reading, well above the roughly 5 arcminutes a whole letter subtends at the 6/6 acuity threshold, because continuous prose demands more margin than identifying an isolated optotype. The other checks come from published guidance and practice. WCAG 2.1 Success Criterion 1.4.12 Text Spacing requires that content stay usable when line height is set to at least 1.5 times the font size, paragraph spacing to 2 times, letter spacing to 0.12em and word spacing to 0.16em, which is where the 1.5 line-height threshold originates. Success Criterion 1.4.4 requires text to remain functional when resized to 200 percent. Note what the standard does not do: WCAG specifies no minimum font size in pixels, so the 16px body recommendation and the audience minimums used here - 18px for older readers and children, 24px for low vision - are conventions from practice, not conformance requirements. The weight penalty reflects that light strokes lose contrast at a given size, and no such number replaces testing with real users on real displays.

Measuring vision by angle rather than by physical size goes back to Herman Snellen, an ophthalmologist in Utrecht, who published his letter chart in 1862. Snellen built his optotypes on a five-by-five grid so that each stroke subtended one arcminute and the whole letter five arcminutes at the reference distance, defining what is still called normal acuity. That framework is why type legibility is discussed in arcminutes at all, and why any size recommendation is meaningless without a stated viewing distance. Applying the principle to accessible documents took much longer. The World Wide Web Consortium launched the Web Accessibility Initiative in 1997, published Web Content Accessibility Guidelines 1.0 in 1999, and released WCAG 2.0 in December 2008, which was also adopted as ISO/IEC 40500 in 2012. WCAG 2.0 deliberately avoided fixed pixel minimums in favour of testable requirements about resizing and contrast, since users control their own rendering. WCAG 2.1, published in June 2018, added Success Criterion 1.4.12 Text Spacing after evidence that users with dyslexia and low vision benefit from adjustable line and letter spacing, along with criteria for reflow and non-text contrast. Legal adoption followed, notably through the European standard EN 301 549 and the accessibility rules attached to Section 508 in the United States, which is why these thresholds now appear in procurement requirements rather than only in design advice.