Readability is how easy wording is to understand; legibility is how easy letters are to see. Learn what each affects and how to check them.
Readability vs legibility comes down to two different questions. Readability asks how easily readers understand the words and sentences. Legibility asks how easily they can see the letters, through typeface, size, spacing, contrast and line length. A formula can estimate the first. Only design choices can improve the second.
Editors, marketers and accessibility reviewers often use "readable" to mean both "easy to understand" and "easy to see." The confusion is understandable, since both affect whether a reader finishes a page. But they are measured differently, fixed by different people, and checked against different standards. A government plain-language review and a print designer's proof check different things, and a page can pass one and fail the other.
This article separates the two. It defines each term, explains what each one affects, shows how to check it, and states where the readability formulas stop being useful. It also maps the relevant WCAG success criteria, with their conformance levels stated exactly, so you know which requirements apply to which problem.
Readability is a property of the language. It describes how much effort the wording takes to understand: how long sentences are, how many syllables words contain, how dense the vocabulary is, and how the ideas are organised. Readability is what people mean when they ask "what is readability" in an editorial context, and it can be estimated numerically.
Legibility is a property of the display. It describes how easily individual characters can be distinguished and seen comfortably. Typeface design, font size, letter and line spacing, the contrast between text and background, and line length all affect it. A typeface with ambiguous letterforms, such as a capital I and a lowercase l that look nearly identical, lowers legibility even when the words are simple.
The practical test is simple. Set a plain-language paragraph in a pale grey, 9-point font on a white background, and it becomes less legible while its readability stays the same. Rewrite a dense legal clause with short sentences and common words, then set it in a clear 11-point sans serif with strong contrast, and both properties improve.
Two terms worth keeping separate in your style guide: "reading level" describes the education level a text requires, and "legibility" describes whether the text can be seen. Treating them as one measure leads to fixes aimed at the wrong problem.
Readability affects comprehension, speed and whether a reader acts on the content. When sentences carry too many clauses or words are unfamiliar, readers slow down, reread and sometimes stop. In patient information, government forms and technical documentation, that friction has consequences beyond annoyance.
Readability is checked with formulas that count measurable features. The most widely used are Flesch Reading Ease, which rewards short sentences and short words, and Flesch-Kincaid Grade Level, which converts the same inputs to a US school grade. How the readability formulas work explains the inputs each one uses. Flesch Reading Ease, for example, combines average sentence length with syllables per word, and every edit that shortens either input pushes the score in the expected direction.
Formulas are useful for comparison and for catching drift, such as a document that gets longer and denser across revisions. They do not measure whether the argument is clear, whether the reader has the background to follow it, or whether terms are defined. For that, you need a human read with a real audience in mind. Readability formulas also depend on how syllables and sentences are counted, so the same text can produce slightly different results across tools. Use one tool consistently when you compare drafts.
Legibility affects whether people can see and process the characters at all. Poor legibility produces fatigue, skipped lines and misreadings, and it hits older readers, people with low vision, and anyone reading on a glare-prone screen hardest.
Font size interacts with line length. Larger type generally needs a wider measure to avoid a choppy reading rhythm, while small type on a wide line makes it hard to find the start of the next line. Typographic guidance commonly recommends roughly 45 to 75 characters per line for continuous body text. Treat that as a convention to test against your own layout, not a rule.
Contrast is one of the most important legibility choices you control, and one of the most often missed: WebAIM's annual Million report finds low-contrast text is the most common accessibility error it detects on home pages. Grey text on a light grey background can look elegant in a design file and be nearly unreadable on a laptop in daylight. Check every text and background pair, including text on images, buttons and hover states.
Spacing matters because crowded lines make it easy to lose your place. Adequate leading (line height), paragraph spacing and letter spacing reduce that problem. Spacing choices also influence how well text survives user-applied overrides, which is why the WCAG text spacing criterion tests it directly (covered below).
A readability formula reads text as a stream of words and punctuation. It has no access to typeface, colour, size or layout. The output is therefore silent on legibility, and this is the single most important point to get right.
Consider three cases. A short, plain sentence set in 6-point pale grey type on a white background will score as very easy to read and be very hard to see. A clear 16-point black sans serif paragraph of dense, jargon-heavy sentences will be easy to see and hard to understand. A web page with generous spacing and strong contrast can still fail a reader who needs a different reading level, because the words themselves are too complex.
So when someone asks "does a readability score measure legibility," the answer is no. A high score tells you the wording is accessible in terms of sentence and word structure. It tells you nothing about whether the page renders legibly on the devices and settings your readers use. Report the two separately: a readability score in your editorial checklist and a contrast, resize and spacing check in your design or QA checklist.
The Web Content Accessibility Guidelines (WCAG) are the most widely referenced standard for digital accessibility. WCAG 2.2 is the current W3C Recommendation; check the W3C site for a later version before citing it in a compliance document. The criteria below are exact, and the conformance levels matter because they determine which requirements a conformance claim must meet.
The first three are legibility criteria. The fourth is a readability criterion, and it sits at Level AAA, so AA conformance does not require it. That split is worth remembering when you scope a project. For a deeper look at how these two sets of requirements overlap in practice, see readability and accessibility under WCAG.
For contrast, test your actual colour pairs with a contrast checker and record the ratio. For resizing, zoom the browser to 200 percent and confirm that no text is clipped, no content overlaps, and no functionality disappears. Text spacing is tested by applying the override values in a browser or bookmarklet and confirming the layout holds. Keep a short record of each check so the results can be reproduced.
Start with the problem you actually have. If readers complain that they cannot understand a document, begin with readability. Run a formula to establish a baseline, then rewrite the longest sentences and the most complex words. What counts as a good readability score gives reasonable target ranges for different audiences, and practical ways to improve your readability score lists the edits that move the numbers most.
If readers complain that they cannot see the text, begin with legibility. Check contrast first, since it is the most commonly detected failure and usually the cheapest to fix. Then test resizing and spacing. Don't attempt to fix a legibility problem by editing words, and don't attempt to fix a readability problem by changing fonts.
For most teams the right order is legibility checks at design and build time, then readability checks at drafting and review time. That way each review examines a property it can actually evaluate.
No. Legibility describes whether individual characters can be told apart and seen comfortably, which depends on typeface, size, contrast and spacing. Readability describes how easily the words and sentences are understood. Text can be highly legible yet hard to read, and the reverse is also possible.
No. Formulas such as Flesch Reading Ease count sentence length and syllables per word. They never see font, size, contrast or spacing. A passage in a tiny, low-contrast font can score as easy while being hard to see. Check visual factors separately with WCAG contrast and resize tests.
WCAG 2.2 success criterion 1.4.3 Contrast (Minimum), a Level AA requirement, asks for a contrast ratio of at least 4.5:1 for normal text and 3:1 for large text. Large text is generally 18 point or 14 point bold. Check each text and background pair with a contrast checker.
Yes, at Level AA. Success criterion 1.4.4 Resize Text requires text to be resizable up to 200 percent without loss of content or functionality. Test it by zooming your browser to 200 percent and checking that nothing is clipped, overlapped or lost.
WCAG 1.4.12 Text Spacing, a Level AA criterion, tests content with line height at least 1.5 times font size, paragraph spacing at least 2 times font size, letter spacing 0.12 times font size and word spacing 0.16 times font size. Content must not lose information or functionality.
Success criterion 3.1.5 Reading Level is Level AAA. It asks that supplemental content or a simpler version be available when text needs reading ability beyond lower secondary education. AA conformance does not require it.
Typographic guidance commonly recommends roughly 45 to 75 characters per line for continuous body text, with narrower lines helping readers track from line to line. This is a practitioner convention rather than a WCAG requirement, so test it on your own layout and audience.
Readability and legibility answer different questions. Readability is about whether the wording is easy to understand, and you can estimate it with formulas that count sentence and word length. Legibility is about whether the letters can be seen comfortably, and you check it through typeface, size, contrast, spacing and line length. A formula score tells you nothing about legibility, so report the two separately. Use WCAG 1.4.3, 1.4.4 and 1.4.12 (all Level AA) for visual checks, and treat 3.1.5 Reading Level (Level AAA) as the reading-level requirement it is. Fix the problem you have, with the tool built to measure it.