Carat Weight Calculator - mm to Carat and Grams

A carat weight calculator is a specialty tool that turns a diamond's millimeter dimensions into an estimated carat weight without putting the stone on a scale, then prints the same weight in grams, milligrams, and ounces. Pick a shape, enter length, width, and depth (or diameter and depth for round brilliants), and the form returns a single carat figure that matches gemology formulas.

• • Estimate weight for an unmounted stone: Measure the diameter of a loose round brilliant with calipers, type the depth, and read the carat weight before an appraisal or price quote.
• • Verify a jeweler's quote: Match the dimensions on a GIA-style diamond report against the calculator's carat result to sanity-check the weight the seller lists.
• • Compare shapes at the same mm size: Switch between princess, emerald, oval, and cushion at the same length x width to see how the carat weight changes shape by shape.
• • Convert carats to grams: Use the gram, milligram, and ounce outputs when the carat weight feeds a recipe, a chemistry unit, or a parcel weight.

Because the metric carat was standardized at 200 milligrams over a century ago, every carat figure is anchored to a fixed gram value, and the gram, milligram, and ounce outputs derive from that single number.

When the carat weight feeds a resale appraisal, the Gold Melt Calculator applies a fineness-adjusted mass calculation to gold scrap and pairs naturally with this diamond weight estimator.

The calculator picks a shape-specific factor, multiplies length, width, and depth (or diameter squared times depth for round), then applies an optional girdle thickness percentage and a shape correction percentage. The result is a carat weight that the calculator also prints as grams, milligrams, and ounces using the 200 mg metric carat standard.

• shape: Selects the shape factor. Round uses diameter and depth; the eleven other shapes use length, width, and depth.
• length: Longest table dimension in mm. Kept as a fallback for switching back to Round.
• width: Shortest table dimension in mm. Kept as a fallback diameter when switching back to Round.
• diameter: Diameter of a round brilliant in mm. Kept as a fallback width when switching to a fancy shape.
• depth: Height from table to culet in mm, used for every shape.
• girdlePct: Girdle thickness as a percentage of the average diameter: 0-1% thin, 1-3% medium, 3-6% thick, 6%+ extra thick. Default 2%.
• correctionPct: Shape-specific correction factor for non-standard proportions, ranging from -5% to +18%.

The shape factor converts a rectangular product of three dimensions into a carat figure. A round brilliant uses 0.0061, a princess uses 0.0083, a heart uses 0.0059, and a marquise looks up its factor by length-to-width ratio. GIA's 4Cs Diamond Cut page groups every non-round cut (princess, marquise, emerald, pear, oval, heart, triangle) as a fancy shape with its own proportions, which is why each fancy cut gets its own per-shape factor in the mm-to-carat math.

A jeweler who prices both metal and stone lots will use the same weight-driven workflow, so the Silver Melt Calculator applies a fineness-adjusted mass calculation to silver scrap and pairs naturally with this diamond weight estimator.

Four ideas explain every number the calculator returns and why the same mm footprint can yield different carat weights across shapes.

For a jeweler who also works with precious metals, the Scrap Gold Calculator applies the same purity-and-weight conversion that the carat-to-gram math applies to gemstones, so the two calculators sit in the same appraisal workflow.

Six short steps take you from millimeter measurements to a carat figure and a parallel gram, milligram, and ounce read-out.

1. 1 Pick the diamond shape: Choose Round or any of the eleven fancy cuts (Princess, Emerald, Oval, Cushion, Marquise, Pear, Heart, Radiant, Asscher, Baguette, Trillion).
2. 2 Enter the table dimensions: For a round brilliant, type the diameter and depth. For a fancy shape, type length, width, and depth from a diamond report or caliper reading.
3. 3 Set the girdle thickness: Use 0-1% for thin, 1-3% for medium, 3-6% for thick, and 6%+ for extra thick. The default of 2% matches a medium girdle on a standard cut.
4. 4 Set the shape correction: Leave at 0 for a standard cut, or enter -5% to +18% for non-standard proportions such as wide wings, pavilion bulge, or wide corners.
5. 5 Read the carat weight: The result panel shows the carat weight rounded to the nearest thousandth of a carat, with the gram, milligram, and ounce values below.
6. 6 Reset to try a new shape: Use the Reset button to restore the default shape and dimensions when switching between loose stones or between round and fancy cuts.

If you are sizing the ring that will hold the stone, the Inches to Ring Size Converter reads finger circumference in millimetres and prints matching US, UK, EU, and Japanese ring sizes.

• • Twelve cuts in one form: Round, princess, emerald, oval, cushion, marquise, pear, heart, radiant, asscher, baguette, and trillion share the same selector.
• • Four weight units at once: Carat, gram, milligram, and ounce outputs are printed in the same result panel, so the same number feeds a diamond quote, a chemistry workbook, and a shipping form.
• • Optional girdle and correction factors: Two percent-range inputs move the estimate from a basic volume figure to a value that matches a grading report.
• • Transparent shape factor: The result panel prints the formula that was used, so a reader can confirm whether diameter squared or length x width was applied.
• • Reset for fast shape switching: The Reset button restores the default shape and dimensions, which makes it quick to compare round brilliants against princess or emerald cuts at the same mm size.

For a buyer or seller who needs the carat number in a different mass unit, the Weight Converter translates grams, kilograms, pounds, ounces, and tons in both directions.

Three inputs change the carat figure and three caveats explain when the mm-to-carat estimate may diverge from a measured scale.

• • The calculator uses standardized formulas calibrated against typical cuts; a diamond with an unusually thick girdle, off-center culet, or asymmetric pavilion can read several hundredths of a carat away from a real scale.
• • Colorless, near-colorless, and fancy-colored diamonds use the same shape factor; the calculator does not adjust for color, clarity, fluorescence, or cut grade.
• • Carat is a unit of mass, not a unit of value. Two diamonds of the same carat weight can differ widely in price because of the other Cs, so the calculator is not a pricing tool.

According to the International Gem Society's diamond carat weight guide, one carat equals 0.2 grams and millimeter measurements only approximate carat weight because the cut changes how much volume fits the same footprint, so the calculator is a planning tool, not a scale replacement. The same source also notes that carat (with a c) measures diamond weight while karat (with a k) measures gold purity: a 1 ct diamond and a 1 g 24K gold setting weigh different amounts and answer different jewelry questions.