Optimal line length API

Converts a target characters-per-line into a px/rem/ch measure using font size and glyph-width factor, and flags whether it sits in the readable 45-75 CPL band (66 ideal). Answers 'How wide should my text column be for 66 characters per line at 16px?', 'Is a 120-character measure too wide?'.

Price$0.04per request

MethodPOST

Route/v1/design/optimal-line-length

StatusLive

MIME typeapplication/json

Rate limit120/minute

Cache0s public

designtypographyline-lengthmeasurecharacters-per-linecplreadabilitymax-width

API URLhttps://x402.hexl.dev/v1/design/optimal-line-length

Integration docs

Example request

{
  "fontSizePx": 16,
  "charactersPerLine": 66
}

Example response

{
  "fontSizePx": 16,
  "charactersPerLine": 66,
  "avgCharWidthEm": 0.5,
  "measurePx": 528,
  "measureRem": 33,
  "measureCh": 66,
  "cssMaxWidth": "66ch",
  "readableBand": {
    "minCpl": 45,
    "idealCpl": 66,
    "maxCpl": 75,
    "inBand": true
  },
  "interpretation": "Measure is within the comfortable reading band."
}

Input schema

{
  "type": "object",
  "required": [
    "fontSizePx"
  ],
  "properties": {
    "fontSizePx": {
      "type": "number",
      "description": "Font size in px",
      "examples": [
        16
      ]
    },
    "charactersPerLine": {
      "type": "number",
      "description": "Target CPL (default 66)",
      "examples": [
        66
      ]
    },
    "avgCharWidthEm": {
      "type": "number",
      "description": "Average glyph width in em (default 0.5)",
      "examples": [
        0.5
      ]
    }
  }
}

Output schema

{
  "type": "object",
  "additionalProperties": true
}