Diamond Weight Calculator
Convert carats ↔ grams, milligrams & points — and estimate carat weight from millimeter size by cut shape. Free, private, runs entirely in your browser.
Exact definitions: 1 ct = 200 mg = 0.2 g and 1 point = 0.01 ct. So 2 ct = 0.4 g = 400 mg = 200 points.
Formula (trade approximation): ct = L × W × depth × k, k = 0.0061 round · 0.0080 princess/emerald · 0.0062 oval · 0.0060 pear · 0.0059 cushion · 0.0057 marquise. Estimate only — real weight varies with girdle, culet and proportions.
Inversion of the round-brilliant formula at the assumed depth. A 1.00 ct round at 62% depth measures about 6.42 mm; at 59% depth the same weight spreads to about 6.53 mm. Fancy shapes need length and width separately — work backwards with the Size → Weight tab.
Worked examples
2 × 0.2 = 0.4 g, 2 × 200 = 400 mg, 2 × 100 = 200 points. So 2 ct = 0.4 g = 400 mg = 200 points.0.75 × 0.2 = 0.15 g; 0.75 × 200 = 150 mg; 0.75 × 100 = 75 points. A 0.75 ct stone is a 75-pointer weighing 0.15 g.6.5² × 4.03 × 0.0061 = 1.0386 ct ≈ 1.04 ct, which is 1.0386 × 0.2 ≈ 0.2077 g. This is why a 6.5 mm round is sold as "about one carat".Round brilliant: diameter → estimated weight
Approximate carat weights for a round brilliant at 62% total depth (formula: D² × 0.62D × 0.0061). Real stones vary — use the calculator above with your stone's measured depth.
| Diameter (mm) | Est. carat (ct) | Est. grams (g) |
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Frequently asked questions
How many grams is 1 carat of diamond?
One metric carat is defined as exactly 200 milligrams, which is 0.2 grams. So 1 carat = 0.2 g, 2 carats = 0.4 g, and 5 carats = 1.0 g. This definition is exact and identical for natural diamonds, lab-grown diamonds, and every other gemstone.
What is a diamond point, and how many points are in a carat?
A point is a jeweler's unit equal to one hundredth of a carat: 1 point = 0.01 ct, so 100 points = 1 carat. A 0.50 ct stone is described as a 50-pointer, and a 0.75 ct stone as a 75-pointer.
How many milligrams is 2 carats?
2 carats = 400 milligrams = 0.4 grams = 200 points, because 1 carat is exactly 200 mg. Multiply any carat weight by 200 to get milligrams.
What is the difference between carat and karat?
Carat (ct) measures gemstone weight: 1 ct = 200 mg. Karat (K or kt) measures gold purity: 24 karat is pure gold. A 2-carat diamond ring describes the stone's weight, while 18-karat gold describes the metal's purity. The two words are unrelated.
How do you estimate a diamond's carat weight from its millimeter size?
For a round brilliant, the trade approximation is: carat = diameter squared x depth x 0.0061, with all measurements in millimeters. A 6.5 mm round with 4.03 mm depth (62 percent) estimates to about 1.04 ct. For fancy shapes use length x width x depth x a shape constant (for example 0.0080 for princess and emerald cuts).
How accurate is a millimeter-to-carat estimate?
It is an approximation, typically within about 5 to 10 percent for well-cut rounds. Actual weight depends on girdle thickness, culet size, and exact proportions. Only a scale gives the true weight; this estimate is for quick reference when only caliper measurements are available.
What total depth percentage should I use for the estimate?
Most round brilliants have a total depth of 59 to 62.5 percent of the diameter. Use 62 percent as a neutral default. Shallow stones (under 58 percent) weigh less than the formula suggests; deep stones (over 64 percent) weigh more.
Does diamond shape change the weight for the same millimeter size?
Yes. For the same face-up area, a princess or emerald cut carries more weight below the girdle than a marquise. The calculator applies a shape constant: 0.0061 for round, 0.0080 for princess and emerald, 0.0062 for oval, 0.0060 for pear, 0.0059 for cushion, and 0.0057 for marquise.
My 6.5 mm diamond weighs 0.97 ct, not 1.04 ct. Why?
The 1.04 ct figure assumes a 62 percent total depth. Your stone is likely cut slightly shallower (around 59 percent), or has a thinner girdle. Enter your stone's actual measured depth to get an estimate that matches its real proportions.
What is the difference between this tool and a general weight converter?
This calculator is gemology-specific: it converts carats, points, milligrams, and grams, and estimates carat weight from millimeter dimensions by cut shape. For everyday mass units such as kilograms, pounds, ounces, stone, or tonnes, use our general weight converter instead.
Is this diamond weight calculator an appraisal?
No. It converts weight units and estimates weight from dimensions. It says nothing about a diamond's value, which depends on cut quality, color, clarity, fluorescence, and market factors. For valuation, consult a qualified gemologist or certified appraiser.
Do two diamonds of the same carat weight look the same size?
Not necessarily. A shallow-cut 1.00 ct round can measure 6.7 mm across and look larger than a deep-cut 1.00 ct at 6.2 mm. Face-up size depends on proportions; carat measures weight only.
How do I convert grams to carats?
Multiply grams by 5, because 1 gram = 5 carats (1 ct = 0.2 g). For example, 0.4 g x 5 = 2 carats.
Do lab-grown diamonds use the same carat weight?
Yes. The metric carat (200 mg) applies identically to natural diamonds, lab-grown diamonds, and diamond simulants such as moissanite and cubic zirconia. Weight units do not depend on origin.
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