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Readability

How easily continuous text can be read, governed mostly by size, line length, leading and contrast.

Definition

Legibility is whether individual characters can be distinguished. Readability is whether continuous text can be read comfortably over time. Both are measurable and both are largely determined by a small number of variables.

Mixed evidence. Real research exists, but findings conflict, effect sizes are small, or the jump from the lab to interface design is contested.

On this page
  1. Definition
  2. The variables that matter, in rough order
  3. Constraining measure
  4. In practice
  5. Sources
  6. Continue from here

The variables that matter, in rough order

  1. 1.Size: 16px minimum for body text on screens. Below that, comprehension and speed both drop for a substantial share of readers.
  2. 2.Line length, or measure: roughly 45 to 75 characters for continuous prose. Long lines make the return sweep unreliable.
  3. 3.Line height: 1.5 or more for body text.
  4. 4.Contrast: 4.5:1 minimum, and comfortably above it for long-form reading.
  5. 5.Alignment: left aligned in left-to-right languages. Justified text on the web creates uneven word spacing.
  6. 6.Paragraph length: short paragraphs are scanned; long ones are skipped.

Constraining measure

css
.prose {
  max-inline-size: 68ch;
  font-size: 1rem;      /* 16px, respects user preference */
  line-height: 1.6;
}

/* Wide containers keep the text column readable */
.article-layout {
  display: grid;
  grid-template-columns: minmax(0, 68ch);
  justify-content: center;
}

The ch unit is approximately the width of the zero character, which makes it a convenient proxy for character count.

In practice

Full-width paragraphs on wide screens

Content sites

Text running the full width of a 1600px viewport reaches well over 150 characters per line and is genuinely hard to read. A max-inline-size on the prose container is a one-line fix.

Sources

Where a source establishes something narrower than the popular reading of it, the note says so.

  • The Elements of Typographic Style

    R. Bringhurst, Hartley and Marks, 1992

    Source of the widely quoted 45 to 75 character measure, from print practice.

  • Reading online text: a comparison of four white space layouts

    M. Chaparro et al., Usability News, 2004

    One of several screen studies producing results that complicate the print guidance.

Continue from here

Each link says what the connection is, so you can tell a principle from an alternative from a thing people mix this up with.

Applies when you are designing

A Mobile App

Short definition

The one-paragraph version, for when that is all you need.

Related concept

Connected closely enough to change how you apply this.