Line length optimizer

Line Length Optimizer

Purpose

Check whether a block of body text sits within a comfortable reading measure, and produce a concrete CSS fix (max-width in ch or px) when it doesn't. Based on the standard typographic range: 45–75 characters per line on desktop, 35–45 on mobile (Bringhurst, Latin — see References).

When To Use

• Reviewing a container/component that holds body copy (article, blog post, card description) for readable width.
• The user pastes a paragraph and asks "is this column too wide?" or gives a max-width/font-size and asks if it's in range.
• Not for headings, buttons, labels, or anything that's one line by design — measure only matters for wrapping prose.

Inputs

One of:

• A CSS snippet or container width + font-size (compute expected characters-per-line), or
• A block of text + a stated or assumed container width, or
• Just a block of text with no width given — in that case, output the recommended max-width rather than judging an existing one.

Target device: desktop, mobile, or both (default: report both).

Outputs

• Verdict: in range / too wide / too narrow, with the measured or estimated characters-per-line.
• A concrete fix: max-width: NNch; (preferred — ch scales with font, unlike a fixed px) or an equivalent px value if the codebase doesn't use ch.
• Change log explaining the math, not just the answer.

Workflow

1. Get characters-per-line.

   • If given a text block: count average characters per line (including spaces) across the paragraph, or estimate from total character count ÷ expected line count if it's reflowing.
   • If given max-width + font-size (+ font-family if known): estimate using ch unit semantics — 1ch ≈ the width of "0" in the given font, so max-width: 65ch targets ~65 characters regardless of font-size. If the input uses px instead of ch, note that the actual character count will vary by font and flag it as a reason to prefer ch.

2. Compare against target range.

   • Desktop: 45–75 characters, 65 is the commonly cited sweet spot (Bringhurst).
   • Mobile: 35–45 characters.
   • Serif body text tolerates the higher end of the range better than sans-serif at small sizes — note this as a soft factor, not a separate hard range.

3. Verdict + fix.

   • In range: confirm, no change needed.
   • Too wide: recommend max-width: 65ch; (or narrower, e.g. 60ch, if the content is dense/technical) on the text container.
   • Too narrow: recommend removing an over-tight max-width or padding, or increasing container width — narrow columns cause more line breaks and more ragged/hyphenation pressure, which interacts with microtypography's Rule 3.
   • If mobile and desktop diverge, recommend a responsive value: either two breakpoint-specific max-width values, or a ch-based value that already scales reasonably since mobile font-sizes are usually smaller too — call out if a media query is actually needed (e.g. desktop uses a much larger font-size, breaking the ch scaling assumption).

4. Report, don't silently edit a stylesheet. Output the CSS as a suggestion in the response; only apply it to a file if the user explicitly asks you to edit that file.

Figma Node Integration

When running with Figma access (Figma MCP / plugin context), read the selected node instead of asking the user to paste text:

• textNode.textAutoResize — determines what "width" even means for this node:
  • NONE or HEIGHT → the node has a fixed width; read textNode.width (or the bounding box) directly and treat it as the measured container width.
  • WIDTH_AND_HEIGHT (auto-width) → there is no fixed measure to check; the box grows with content. Flag this instead of computing a verdict — recommend switching to HEIGHT (auto-height, fixed width) if the intent is a readable paragraph column, since auto-width text has no line wrap at all.
• textNode.fontSize and textNode.fontName.family — needed to convert a pixel width into an estimated character count (characters-per-line ≈ width in px ÷ average glyph width, which scales with fontSize and varies by typeface — monospace and condensed faces need a different divisor than a default sans).
• Action back to Figma: if the fix is "too wide," the agent may resize the node's width directly (set textNode.resize(newWidth, textNode.height) or the equivalent plugin API call) to the pixel width corresponding to ~65 characters at the node's current fontSize/font — but only when the user has asked for a direct fix, not as a silent default. Otherwise, report the recommended width and let the user apply it.

Quality Checklist

• Character count is stated explicitly (measured or estimated), not just "looks too wide."
• Recommendation uses ch unless the codebase's existing convention is px/rem — match existing convention if shown in the input.
• Desktop and mobile both addressed, even if the answer is "same value works for both, here's why."
• Didn't touch headings, labels, or non-wrapping UI text — flag if the user's target isn't actually body prose.
• If a fix is suggested for a real file, it's proposed, not silently applied, unless the user explicitly asked for a direct edit.
• Noted the interaction with microtypography Rule 3 when the fix affects ragged-edge risk (narrower column → more hyphenation/rag pressure).

References

• Bringhurst, R. (2012). The Elements of Typographic Style (4th ed.). Hartley & Marks — standard measure: 45–75 characters per line, 66 as the classic ideal.
• Hochuli, J. (2008). Detail in Typography. Hyphen Press — how the eye scans a line, and why an optimal word count per line reduces reading fatigue.
• Latin, M. (2017). Better Web Typography for a Better Web. — practical ch-based implementation for web body text, and the mobile 35–45 character range.
• Figma Plugin API Docs: TextNode.textAutoResize.
• Related skill: microtypography — apply after fixing measure, since a corrected column width changes where ragged-edge and hyphenation risks actually occur.