Font Size Accessibility Checker
Use our free Font size accessibility Calculator to learn and practice. Get step-by-step solutions with explanations and examples.
Formula
Visual Angle (arcmin) = atan(Font Height mm / Distance mm) x 3438
The visual angle determines how large text appears on the retina at a given viewing distance. Font height in mm is calculated from pixel size (px x 0.2646mm at 96dpi). A minimum visual angle of 12 arcminutes is needed for comfortable reading. The accessibility score combines visual angle, WCAG requirements, audience needs, and text spacing checks.
Worked Examples
Example 1: Corporate Website Body Text
Problem: A corporate website uses 14px body text with font-weight 400, line-height 1.4, viewed at 65cm on desktop. Evaluate its accessibility for a general audience.
Solution: Font size: 14px (below recommended 16px minimum)\nLine height: 1.4 (below WCAG 1.5 minimum)\nVisual angle: atan((14 x 0.2646) / 650) x 60 = 10.6 arcminutes (below 12 min)\nWCAG minimum (12px): PASS\nRecommended (16px): FAIL\nAudience minimum (16px): FAIL\nLine height (1.5): FAIL\nScore: 15 + 0 + 0 + 0 + 0 + 10 = 25/100
Result: Score: 25/100 | WCAG Level: Fail | Increase to 16px with 1.5 line-height
Example 2: Accessible Design for Elderly Users
Problem: An online pharmacy targets elderly users (65+). They plan to use 20px font, weight 400, line-height 1.6, viewed at 55cm. Is this accessible?
Solution: Font size: 20px (above general 16px minimum)\nTarget audience: elderly (minimum 18px)\nLine height: 1.6 (above WCAG 1.5 minimum)\nVisual angle: atan((20 x 0.2646) / 550) x 60 = 16.6 arcminutes (above 12 min)\nWCAG minimum: PASS\nRecommended: PASS\nAudience minimum (18px): PASS\nLine height: PASS\nVisual angle: PASS\nWeight check: PASS\nScore: 15 + 20 + 20 + 20 + 15 + 10 = 100/100
Result: Score: 100/100 | WCAG Level: AAA | Excellent accessibility for elderly users
Frequently Asked Questions
What is the minimum font size for web accessibility?
While WCAG (Web Content Accessibility Guidelines) does not specify an absolute minimum font size in pixels, the widely accepted best practice is 16px (1rem) for body text on the web. This is the default browser font size and provides comfortable readability for most users at typical screen viewing distances of 50-70cm. The WCAG Success Criterion 1.4.4 requires that text can be resized up to 200% without loss of content or functionality. For users with low vision, the minimum recommended size is 24px. Never use text smaller than 12px for any content, and text below 14px should be limited to captions and legal disclaimers.
How does WCAG address font size and text spacing?
WCAG 2.1 Success Criterion 1.4.12 (Text Spacing) requires that content and functionality remain intact when users override text spacing properties. Specifically, line height must support at least 1.5 times the font size, paragraph spacing must support 2 times the font size, letter spacing must support 0.12 times the font size, and word spacing must support 0.16 times the font size. SC 1.4.4 (Resize Text) requires text to be resizable up to 200% without assistive technology. SC 1.4.8 (Visual Presentation) recommends line lengths of no more than 80 characters and adequate line spacing for enhanced accessibility.
How does viewing distance affect minimum font size requirements?
Viewing distance is crucial for determining readable font sizes because it directly affects the visual angle subtended by text on the retina. Text that appears readable at arm length (50-60cm for desktops) becomes unreadable at greater distances. For mobile phones typically held at 25-35cm, slightly smaller sizes can work. For presentations or kiosk displays viewed at 100-200cm, significantly larger text is needed. The minimum comfortable visual angle for reading is approximately 12 arcminutes. Font Size Accessibility Checker uses this principle to evaluate whether your font size creates a sufficient visual angle at your specified viewing distance for comfortable sustained reading.
Why do different audiences need different minimum font sizes?
Visual acuity, cognitive processing speed, and familiarity with digital interfaces vary significantly across user populations. Elderly users (65+) typically experience presbyopia and reduced contrast sensitivity, requiring minimum 18px font sizes. Children are still developing reading skills and benefit from larger text at 18px minimum. Users with low vision conditions need 24px or larger as a starting point. The general adult population can comfortably read 16px body text on screens. Mobile-first designs should use at least 16px since phone screens are viewed at closer distances but have higher pixel density. Always design for your most challenged user group.
How does font weight affect readability and accessibility?
Font weight directly impacts character stroke visibility and contrast against the background. Lighter weights (100-300) have thinner strokes that are harder to distinguish, especially at small sizes, on low-contrast backgrounds, or on low-resolution displays. A font at 16px with weight 300 (light) may have the same effective readability as a 14px font at weight 400 (regular). WCAG contrast requirements assume regular weight text. When using light or thin fonts, designers should increase the font size by 2-4px to compensate for the reduced stroke visibility. Bold text (600-700) can tolerate slightly smaller sizes due to its increased stroke presence.
How do I test font size accessibility on my website?
Several methods exist for testing font size accessibility. Browser developer tools allow you to inspect computed font sizes and simulate vision impairments. The WAVE accessibility evaluation tool highlights text size issues. Testing with browser zoom at 200% verifies WCAG SC 1.4.4 compliance. Use the browser built-in accessibility audit (Lighthouse) for automated checks. Manual testing should include resizing text using browser settings, verifying content remains functional at 200% zoom, and testing with screen readers. Real user testing with individuals who have visual impairments provides the most valuable feedback, as automated tools cannot fully capture the reading experience.