{
  "id": "gemstone-weight",
  "version": "ecf89c67ed9c",
  "status": "published",
  "name": "Gemstone Weight Calculator",
  "question": "What is my gemstone weight?",
  "summary": "Estimates a gemstone’s weight in carats from its measurements in millimeters, its cut and its specific gravity, with the International Gem Society shape formulas.",
  "category": "everyday-life",
  "subcategory": "money-and-shopping",
  "url": "https://www.acalculator.org/everyday-life/gemstone-weight-calculator",
  "markdown": "https://www.acalculator.org/everyday-life/gemstone-weight-calculator.md",
  "kind": "function",
  "method": "Rounds, ovals and squares: diameter² × depth × SG × factor. Rectangles, pears, hearts, marquises, triangles and cabochons: length × width × depth × SG × factor.",
  "assumptions": [
    "The result is an estimate for a well-cut stone. IGS gives corrections for thick girdles, bulging pavilions and other cuts that this calculator does not apply.",
    "Measurements are in millimeters; the result is in metric carats (1 ct = 0.2 g)."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "shape": {
        "title": "Cut",
        "description": "The shape and cut of the gem.",
        "type": "string",
        "enum": [
          "round",
          "oval",
          "square",
          "square-cushion",
          "square-step",
          "square-cut-corners",
          "rect-cushion",
          "rect",
          "rect-step",
          "rect-cut-corners",
          "pear",
          "heart",
          "marquise",
          "triangle",
          "cab-high",
          "cab-low"
        ]
      },
      "dia": {
        "title": "Diameter (mm)",
        "description": "The width across a round stone, in millimeters.",
        "type": "number",
        "minimum": 0.1,
        "maximum": 1000
      },
      "l": {
        "title": "Length (mm)",
        "description": "The longest measurement across the top, in millimeters.",
        "type": "number",
        "minimum": 0.1,
        "maximum": 1000
      },
      "w": {
        "title": "Width (mm)",
        "description": "The measurement across the top at right angles to the length, in millimeters.",
        "type": "number",
        "minimum": 0.1,
        "maximum": 1000
      },
      "diag": {
        "title": "Diagonal (mm)",
        "description": "The measurement across the top halfway between the length and the width (corner to corner on a square), in millimeters.",
        "type": "number",
        "minimum": 0.1,
        "maximum": 1000
      },
      "d": {
        "title": "Depth (mm)",
        "description": "From the top (table) to the bottom point (culet), in millimeters.",
        "type": "number",
        "minimum": 0.1,
        "maximum": 1000
      },
      "gem": {
        "title": "Gem",
        "description": "The gem, for its specific gravity (density compared with water).",
        "type": "string",
        "enum": [
          "diamond",
          "corundum",
          "emerald",
          "amethyst",
          "quartz",
          "custom"
        ]
      },
      "sg": {
        "title": "Specific gravity",
        "description": "The gem’s specific gravity, from a gem reference.",
        "type": "number",
        "minimum": 0.5,
        "maximum": 25
      }
    }
  },
  "outputs": {
    "carats": {
      "label": "Estimated weight",
      "description": "The IGS estimate in carats.",
      "format": "number"
    },
    "grams": {
      "label": "Weight in grams",
      "description": "Carats × 0.2 (a metric carat is 200 mg).",
      "format": "number"
    },
    "size": {
      "label": "Diameter used (mm)",
      "description": "The diameter in the formula, for rounds, ovals and squares.",
      "format": "number"
    },
    "factor": {
      "label": "Shape factor",
      "description": "The IGS factor for the cut.",
      "format": "number"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "shape": "round",
      "dia": 6.5,
      "l": 8,
      "w": 6,
      "diag": 7,
      "d": 4,
      "gem": "diamond",
      "sg": 3.52
    },
    "outputs": {
      "carats": 1.070784,
      "grams": 0.2141568,
      "size": 6.5,
      "factor": 0.0018
    },
    "text": "The gem weighs about 1.07 carats."
  },
  "examples": [
    {
      "given": {
        "shape": "round",
        "dia": 6.5,
        "d": 4,
        "gem": "diamond"
      },
      "expect": {
        "carats": 1.070784,
        "grams": 0.2141568,
        "size": 6.5,
        "factor": 0.0018
      },
      "source": "hand calculation in content.mdx: 6.5 × 6.5 × 4 × 3.52 × 0.0018 = 1.070784 ct; International Gem Society, How to Estimate Gem Weight in a Jewelry Setting (shape factors), https://www.gemsociety.org/article/estimating-gem-weight; GIA Gem Encyclopedia (specific gravity: diamond 3.52, ruby and sapphire 4.00, emerald 2.72, amethyst 2.66), https://www.gia.edu/diamond"
    },
    {
      "given": {
        "shape": "oval",
        "l": 8,
        "w": 6,
        "diag": 7,
        "d": 4,
        "gem": "corundum"
      },
      "expect": {
        "carats": 1.568,
        "size": 7
      },
      "source": "hand calculation in content.mdx: diameter = (7 + 8 + 6) ÷ 3 = 7; 7 × 7 × 4 × 4.00 × 0.0020 = 1.568 ct; International Gem Society, How to Estimate Gem Weight in a Jewelry Setting (shape factors), https://www.gemsociety.org/article/estimating-gem-weight; GIA, Ruby (SG 4.00), https://www.gia.edu/ruby"
    },
    {
      "given": {
        "shape": "rect-step",
        "l": 9,
        "w": 7,
        "d": 5,
        "gem": "emerald"
      },
      "expect": {
        "carats": 2.142
      },
      "source": "hand calculation in content.mdx: 9 × 7 × 5 × 2.72 × 0.0025 = 2.142 ct; International Gem Society, How to Estimate Gem Weight in a Jewelry Setting (shape factors), https://www.gemsociety.org/article/estimating-gem-weight; GIA, Emerald (SG 2.72), https://www.gia.edu/emerald"
    },
    {
      "given": {
        "shape": "rect-cushion",
        "l": 10,
        "w": 8,
        "d": 6,
        "gem": "custom",
        "sg": 3.1
      },
      "expect": {
        "carats": 3.31452,
        "size": 9
      },
      "source": "hand calculation in content.mdx: diameter = (10 + 8) ÷ 2 = 9; 9 × 9 × 6 × 3.1 × 0.0022 = 3.31452 ct; International Gem Society, How to Estimate Gem Weight in a Jewelry Setting (shape factors), https://www.gemsociety.org/article/estimating-gem-weight"
    }
  ],
  "sources": [
    "International Gem Society, How to Estimate Gem Weight in a Jewelry Setting (formulas and shape factors for faceted gems and cabochons; girdle and pavilion corrections; updated 2023-04-26). https://www.gemsociety.org/article/estimating-gem-weight (retrieved 2026-10-19)",
    "GIA Gem Encyclopedia: diamond (specific gravity 3.52), https://www.gia.edu/diamond; ruby (4.00), https://www.gia.edu/ruby; sapphire (4.00), https://www.gia.edu/sapphire; emerald (2.72), https://www.gia.edu/emerald; amethyst (2.66), https://www.gia.edu/amethyst (retrieved 2026-10-19)",
    "International Gem Society, Specific Gravity Values of Selected Gems (quartz 2.65). https://www.gemsociety.org/article/select-gems-ordered-density/ (retrieved 2026-10-19)",
    "NIST Special Publication 811 (2008), Appendix B.8: carat, metric = 2 × 10⁻⁴ kg exactly. https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-19)"
  ],
  "related": [
    "ring-size"
  ],
  "changelog": []
}