{
  "id": "ring-size",
  "version": "92e58a474ae7",
  "status": "published",
  "name": "Ring Size Calculator",
  "question": "What is my ring size?",
  "summary": "Finds the US ring size and the ISO ring size from the inside diameter of a ring or the circumference of a finger, or the diameter and circumference of a US size.",
  "category": "everyday-life",
  "subcategory": "clothing",
  "url": "https://www.acalculator.org/everyday-life/ring-size-calculator",
  "markdown": "https://www.acalculator.org/everyday-life/ring-size-calculator.md",
  "kind": "function",
  "method": "Inside diameter (mm) = 11.63 + 0.8128 × US size; US size = (diameter − 11.63) ÷ 0.8128; ISO size = inside circumference (mm) = π × diameter.",
  "assumptions": [
    "US and Canadian sizes follow the common scale: size 0 is 11.63 mm (0.458 in) inside, and each whole size adds 0.032 in (0.8128 mm) to the diameter.",
    "A finger circumference is taken as the ring’s inside circumference; the ISO size is that circumference in millimetres.",
    "Jewelers’ mandrels and ring makers differ slightly; try a ring on before you buy."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "by": {
        "title": "I know the",
        "description": "What you measured: the inside diameter of a ring that fits, the circumference of your finger, or a US size.",
        "type": "string",
        "enum": [
          "diameter",
          "circumference",
          "us"
        ]
      },
      "d": {
        "title": "Inside diameter",
        "description": "The inside diameter of a ring that fits, edge to edge.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "length",
        "minimum": 0.001,
        "maximum": 0.2
      },
      "c": {
        "title": "Finger circumference",
        "description": "The distance around the finger where the ring sits, measured with a strip of paper or string.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "length",
        "minimum": 0.001,
        "maximum": 0.2
      },
      "us": {
        "title": "US size",
        "description": "A US or Canadian ring size, from 0 to 16 in steps such as 7, 7.25, 7.5.",
        "type": "number",
        "minimum": 0,
        "maximum": 16
      }
    }
  },
  "outputs": {
    "nearest": {
      "label": "US ring size",
      "description": "The US and Canadian size to the nearest quarter size.",
      "format": "number"
    },
    "size": {
      "label": "Exact US size",
      "description": "The US size before rounding: (inside diameter in mm − 11.63) ÷ 0.8128.",
      "format": "number"
    },
    "iso": {
      "label": "ISO size",
      "description": "The ISO 8653 size: the inside circumference in millimetres.",
      "format": "number"
    },
    "diameterMm": {
      "label": "Inside diameter (mm)",
      "description": "The inside diameter of the ring in millimetres.",
      "format": "number"
    },
    "diameterIn": {
      "label": "Inside diameter (in)",
      "description": "The inside diameter in inches: millimetres ÷ 25.4.",
      "format": "number"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "by": "diameter",
      "d": 0.01732,
      "c": 0.0544,
      "us": 7
    },
    "outputs": {
      "nearest": 7,
      "size": 7.000492125984252,
      "iso": 54.412384760175215,
      "diameterMm": 17.32,
      "diameterIn": 0.6818897637795276
    },
    "text": "An inside diameter of 17.32 mm is US ring size 7 (ISO 54.4)."
  },
  "examples": [
    {
      "given": {
        "by": "diameter",
        "d": 0.01732
      },
      "expect": {
        "nearest": 7,
        "size": 7.000492125984252,
        "iso": 54.412384760175215
      },
      "source": "Ring size, Wikipedia: North American size to inside diameter d = 0.8128 × s + 11.63 mm (https://en.wikipedia.org/wiki/Ring_size, retrieved 2026-10-01); ISO 8653:2016 defines the size as the inside circumference in mm (https://www.iso.org/standard/66185.html); Python 3 cross-check"
    },
    {
      "given": {
        "by": "us",
        "us": 7
      },
      "expect": {
        "diameterMm": 17.3196,
        "iso": 54.411128123113784
      },
      "source": "Ring size, Wikipedia (https://en.wikipedia.org/wiki/Ring_size): d = 0.8128 × 7 + 11.63 = 17.3196 mm; hand calculation in content.mdx: circumference π × 17.3196 = 54.41 mm",
      "tolerance": 1e-12
    },
    {
      "given": {
        "by": "circumference",
        "c": 0.0544
      },
      "expect": {
        "nearest": 7,
        "size": 6.995641988678903
      },
      "source": "Ring size, Wikipedia (https://en.wikipedia.org/wiki/Ring_size); ISO 8653:2016 (size = inside circumference in mm); hand calculation in content.mdx: 54.4 ÷ π = 17.316 mm, (17.316 − 11.63) ÷ 0.8128 = 6.996",
      "tolerance": 1e-12
    }
  ],
  "sources": [
    "Ring size, Wikipedia: North American sizes, d = 0.8128 × size + 11.63 mm (whole sizes differ by 0.032 in of inside diameter), and ISO 8653:2016 defines a ring size as the inside circumference in millimetres, retrieved 2026-10-01. https://en.wikipedia.org/wiki/Ring_size",
    "ISO 8653:2016, Jewellery — Ring-sizes — Definition, measurement and designation (ring size = inside circumference in mm). https://www.iso.org/standard/66185.html"
  ],
  "related": [
    "bra-size"
  ],
  "changelog": []
}