Diamond Carat to Millimeter Sizes
Jewelry & Precious Metals · 9 specs
Direct Answer
Diamond carat weight is standardized under the Gemological Institute of America (GIA) where 1.00 metric carat equals exactly 0.200 grams (200 milligrams / 100 points). For an ideal 'Triple Excellent' round brilliant cut, carat weight translates into exact face-up millimeter diameters, from 0.10 ct (3.0 mm) to 3.00 ct (9.4 mm).
Source: GIA (Gemological Institute of America) Diamond Grading System / ISO 24016 · Last reviewed Aug 28, 2026
Specs
| Governing Standards | GIA / ISO 24016 (Jewellery — Diamond grading) / CIBJO Diamond Book |
| Carat Weight Definition | 1.00 Carat (1.00 ct) = Exactly 0.200 grams (200 mg / 100 Points) |
| 0.25 Carat (1/4 ct / 25 Pts) | Diameter: 4.1 mm ± 0.1 mm | Surface Area: ~13.2 mm² | Subtle accent solitaire |
| 0.50 Carat (1/2 ct / 50 Pts) | Diameter: 5.1 mm ± 0.1 mm | Surface Area: ~20.4 mm² | Classic balanced solitaire |
| 0.75 Carat (3/4 ct / 75 Pts) | Diameter: 5.8 mm ± 0.1 mm | Surface Area: ~26.4 mm² |
| 1.00 Carat (100 Points) | Diameter: 6.5 mm ± 0.1 mm | Surface Area: ~33.2 mm² | Universal engagement benchmark |
| 1.50 Carat (150 Points) | Diameter: 7.4 mm ± 0.1 mm | Surface Area: ~43.0 mm² |
| 2.00 Carat (200 Points) | Diameter: 8.1 mm ± 0.1 mm | Surface Area: ~51.5 mm² | Over 50% larger visual spread than 1.0ct |
| 3.00 Carat (300 Points) | Diameter: 9.4 mm ± 0.1 mm | Surface Area: ~69.4 mm² |
Round Brilliant Diamond Carat Weight to Millimeter Diameter Matrix
| Carat Weight (ct) | Weight in Points | Weight in Grams | Face-Up Diameter (mm) | Face-Up Surface Area (mm²) | Visual Size Increase vs 1.0ct |
|---|---|---|---|---|---|
| 0.10 ct | 10 Points | 0.020 g | 3.0 mm | 7.1 mm² | — |
| 0.25 ct | 25 Points | 0.050 g | 4.1 mm | 13.2 mm² | -60% |
| 0.50 ct | 50 Points | 0.100 g | 5.1 mm | 20.4 mm² | -39% |
| 0.75 ct | 75 Points | 0.150 g | 5.8 mm | 26.4 mm² | -20% |
| 1.00 ct | 100 Points | 0.200 g | 6.5 mm (0.256") | 33.2 mm² | Benchmark (100%) |
| 1.25 ct | 125 Points | 0.250 g | 7.0 mm | 38.5 mm² | +16% |
| 1.50 ct | 150 Points | 0.300 g | 7.4 mm | 43.0 mm² | +30% |
| 1.75 ct | 175 Points | 0.350 g | 7.8 mm | 47.8 mm² | +44% |
| 2.00 ct | 200 Points | 0.400 g | 8.1 mm (0.319") | 51.5 mm² | +55% |
| 2.50 ct | 250 Points | 0.500 g | 8.8 mm | 60.8 mm² | +83% |
| 3.00 ct | 300 Points | 0.600 g | 9.4 mm (0.370") | 69.4 mm² | +109% |
| 4.00 ct | 400 Points | 0.800 g | 10.2 mm | 81.7 mm² | +146% |
| 5.00 ct | 500 Points | 1.000 g | 11.0 mm | 95.0 mm² | +186% |
Why Carat Measures Weight, Not Direct Size (The 'Cut Score' Trap)
Because carat is a unit of mass (1 ct = 0.2 g), a poorly cut diamond with a deep pavilion ('steep and deep') hides excess weight at the bottom of the setting, resulting in a 1.00 ct diamond with a face-up diameter of only 6.1 mm (looking like a 0.85 ct stone). An Ideal / Excellent cut ensures the diamond reaches its full 6.5 mm spread with maximum light reflection.