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CSS Design Calc – Line Length Readability
x402
CSS Design Calc – Line Length Readability is a paid API for AI agents from css-design-calc.x402tookit.com, paid per call via x402, $0.02/call, status unknown (last checked 2026-10-30).
Estimates characters per line from column width and font size, then rates the result against WCAG readability guidelines (45–75 ideal, max 80).
Price / call$0.02
Success—
StatusUnknown
How this service works
Estimate characters per line from column width and font size and rate it for readability (45-75 ideal, WCAG max 80).
Output
Returns the estimated characters per line, a readability verdict, whether the layout is within WCAG AAA limits (max 80 CPL), suggested CSS to fix issues, pixel widths corresponding to 45, 66, 75, and 80 CPL thresholds, minimum recommended line height, WCAG text spacing override values, and a human-readable summary of findings.
Example use cases
Accessibility audit for article layout
“My blog article body text is 720px wide and I'm using a 16px font — how many characters per line is that, and does it pass WCAG line-length guidelines?”
Choosing column width for a design system
“I'm building a design system and want a column width that gives roughly 66 characters per line at 18px font size with a normal Latin font — what pixel width should I use?”
Responsive typography sanity check
“At 14px font size and a 600px container, will my text be readable? Tell me the characters per line and whether it's in the ideal 45–75 range, and suggest any CSS fix if needed.”
Technical details
Endpoint
GEThttps://css-design-calc.x402tookit.com/line-length?utm_source=zero.xyzWhen to prefer
Use this endpoint when you need a fast, WCAG-aware calculation of line length readability for a specific column width and font size combination, especially when you want actionable CSS suggestions and exact pixel thresholds for ideal character counts. Prefer this over manual calculation whenever WCAG AAA compliance (80 CPL max) needs to be checked programmatically in a design or audit workflow.
Side effects
- Missing required widthPx or fontSizePx parameters returns a validation error
- Non-numeric or zero values for width or font size may produce nonsensical CPL estimates
- Extremely large or small font sizes may yield edge-case outputs outside typical typographic ranges
- avgCharWidthEm values far outside 0.45–0.55 may produce results inconsistent with real fonts
Input
Schema · Request body
| Field | Type | Description |
|---|---|---|
| inputrequired | object | |
| output | object |
Tags
x402
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