# CSS Design Calc – Line Length Readability

> 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).

## Facts

- Endpoint: GET https://css-design-calc.x402tookit.com/line-length?utm_source=zero.xyz
- Price: $0.02/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-30
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/css-design-calc-line-length-readability-dce81b74
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_fkiWnk4Eesa80o0498Jqi

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability css-design-calc-line-length-readability-dce81b74
```

Example prompt: Can you check how many characters per line I'll get in a 680px wide text column at 18px font size, and tell me if that's within WCAG readability guidelines?

## When to prefer this

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.

## Known failure modes

- 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

## 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.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method"
   ],
   "properties": {
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "GET"
     ],
     "type": "string"
    },
    "queryParams": {
     "type": "object",
     "required": [
      "widthPx",
      "fontSizePx"
     ],
     "properties": {
      "widthPx": {
       "type": "number",
       "description": "Text column width in px"
      },
      "fontSizePx": {
       "type": "number",
       "description": "Font size in px"
      },
      "avgCharWidthEm": {
       "type": "number",
       "description": "Average character width in em; 0.5 is typical for Latin text (default 0.5; 0.45 condensed, 0.55 wide)"
      }
     }
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object",
     "properties": {
      "asOf": {
       "type": [
        "null",
        "string"
       ]
      },
      "source": {
       "type": [
        "null",
        "string"
       ]
      },
      "reasons": {
       "type": [
        "array",
        "null"
       ],
       "items": {
        "type": "string"
       }
      },
      "summary": {
       "type": [
        "null",
        "string"
       ]
      },
      "verdict": {
       "type": [
        "null",
        "string"
       ]
      },
      "widthPx": {
       "type": [
        "null",
        "number"
       ]
      },
      "degraded": {
       "type": [
        "boolean",
        "null"
       ]
      },
      "disclaimer": {
       "type": [
        "null",
        "string"
       ]
      },
      "fontSizePx": {
       "type": [
        "null",
        "number"
       ]
      },
      "notChecked": {
       "type": [
        "array",
        "null"
       ],
       "items": {
        "type": "string"
       }
      },
      "suggestedCss": {
       "type": [
        "null",
        "string"
       ]
      },
      "widthFor45Px": {
       "type": [
        "null",
        "number"
       ]
      },
      "widthFor66Em": {
       "type": [
        "null",
        "number"
       ]
      },
      "widthFor66Px": {
       "type": [
        "null",
        "number"
       ]
      },
      "widthFor75Px
… (truncated)
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/css-design-calc-line-length-readability-dce81b74/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
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