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About this tool
Compare the same private text across local system font stacks, size, measure, weight, and spacing settings without declaring a universal winner.
The Font Legibility Comparator renders your own text twice, side by side, under two independent typography settings — font stack, size, weight, line-height, letter-spacing, word-spacing and measure — so you can judge legibility on real content instead of a specimen sheet. A one-click 'Apply spacing stress to B' button pushes View B to the WCAG 2.1 Text Spacing (1.4.12) test values: line-height at least 1.5, letter-spacing at least 0.12em, word-spacing at least 0.16em and paragraph spacing at least 2em, so you can compare it directly against View A's normal spacing. It deliberately never declares a winner or a pass — the exported JSON records the two settings and their deltas, and leaves the judgement to you and your readers.
Open Font Legibility Comparator on AltFTool — it loads instantly in your browser.
Paste your own passage into "Private reading sample", capped at 20,000 characters, with the counter below reporting words, paragraphs, detected script groups and characters used.
Adjust View A settings and View B settings independently: Local font stack (System UI sans, Humanist sans fallback, Neutral sans fallback, Reading serif fallback or Monospace fallback), Font size in px, Line height, Letter spacing and Word spacing in em, Line measure in ch, Font weight from 400 Regular to 700 Bold, and Paragraph spacing.
Press "Apply spacing stress to B" to push that view to the WCAG test values, "Swap views" to compare the other way round, then "Download settings-only report" to save font-legibility-comparison.json, which excludes your sample text and your preference.
Size, weight, line-height, letter-spacing, word-spacing and measure all vary independently per side, so you can isolate which one is doing the work.
One button pushes a view to the 1.4.12 test values, which is how you find text that breaks under a reader's own spacing overrides.
No readability score is invented and no winner is declared; the report records both configurations and the differences between them for you to act on.
Success Criterion 1.4.12 Text Spacing: line height at least 1.5 times the font size, spacing after paragraphs at least 2 times the font size, letter spacing at least 0.12em and word spacing at least 0.16em. Content should stay readable with no loss of function when a reader forces those values — they are a robustness test, not a required design.
No, and that is deliberate. Legibility depends on the reader, the language, the screen and the lighting, so the tool shows both settings on your own text and exports the configurations without a score or a winner. Test with the people who will actually read it.
Five local stacks that need no download: system UI sans, a humanist fallback (Verdana, Tahoma), a neutral fallback (Arial, Helvetica), a reading serif (Georgia, Times New Roman) and a monospace stack. No external font is requested, so nothing about your text or session is sent to a font host.
No. The JSON contains the two normalised settings, their numeric deltas and counts of characters, words, paragraphs and scripts detected — but not the text itself and not the side you preferred. Input is capped at 20,000 characters and anything beyond that is flagged as truncated.