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Stone size on hand: mm to carat

Convert a stone's size in millimetres to its weight in carats and back, for round, oval, cushion, princess, emerald, pear and marquise cuts, in diamond or a coloured stone, and see it to scale on a finger of your ring size.

A 1 carat round diamond is 6.45 mm across, a 6.5 mm round is 1.02 ct, and a 1 carat oval is about 7.88 × 5.84 mm.

Drawn to scale: the finger is 16.51 mm wide, the inside diameter of the ring size; the bar is 10 mm.

Weight: 1.02 ct
Size: 6.50 mm
Depth: 3.96 mm
Specific gravity: 3.52
Across the finger: 39% of 16.51 mm

Estimated from the cut's usual proportions with the trade's formulas, scaled by the stone's specific gravity from Design Facts.

Diamond size chart: carat to mm

The length and width of a diamond of each weight in the usual proportions of its cut. Coloured stones of the same size weigh more or less; the table below compares them.

Diamond sizes in mm by carat weight and cut
Carat ct	Round	Oval	Cushion	Princess	Emerald cut	Pear	Marquise
0.25	4.07 mm	4.97 × 3.68 mm	3.76 × 3.58 mm	3.50 mm	4.55 × 3.03 mm	5.39 × 3.59 mm	6.60 × 3.30 mm
0.5	5.12 mm	6.26 × 4.63 mm	4.73 × 4.51 mm	4.42 mm	5.73 × 3.82 mm	6.79 × 4.52 mm	8.31 × 4.16 mm
0.75	5.86 mm	7.16 × 5.31 mm	5.42 × 5.16 mm	5.05 mm	6.56 × 4.37 mm	7.77 × 5.18 mm	9.52 × 4.76 mm
1	6.45 mm	7.88 × 5.84 mm	5.96 × 5.68 mm	5.56 mm	7.22 × 4.81 mm	8.55 × 5.70 mm	10.48 × 5.24 mm
1.25	6.95 mm	8.49 × 6.29 mm	6.42 × 6.12 mm	5.99 mm	7.78 × 5.18 mm	9.21 × 6.14 mm	11.28 × 5.64 mm
1.5	7.39 mm	9.02 × 6.68 mm	6.83 × 6.50 mm	6.37 mm	8.26 × 5.51 mm	9.79 × 6.52 mm	11.99 × 6.00 mm
2	8.13 mm	9.93 × 7.36 mm	7.51 × 7.16 mm	7.01 mm	9.10 × 6.06 mm	10.77 × 7.18 mm	13.20 × 6.60 mm
2.5	8.76 mm	10.70 × 7.93 mm	8.09 × 7.71 mm	7.55 mm	9.80 × 6.53 mm	11.60 × 7.74 mm	14.22 × 7.11 mm
3	9.31 mm	11.37 × 8.42 mm	8.60 × 8.19 mm	8.02 mm	10.41 × 6.94 mm	12.33 × 8.22 mm	15.11 × 7.55 mm
4	10.24 mm	12.51 × 9.27 mm	9.47 × 9.01 mm	8.83 mm	11.46 × 7.64 mm	13.57 × 9.05 mm	16.63 × 8.31 mm
5	11.03 mm	13.48 × 9.99 mm	10.20 × 9.71 mm	9.51 mm	12.34 × 8.23 mm	14.62 × 9.75 mm	17.91 × 8.96 mm

Round diamonds: mm to carat

Centre stone sizes of a round brilliant at 61% depth, with its depth in mm. For melee and pavé stones under 3 mm, see the gem and metal weight reference.

Round brilliant diamond diameter in mm and carat weight
Diameter mm	Carat ct	Depth mm
3.0	0.10	1.83
3.5	0.16	2.13
4.0	0.24	2.44
4.5	0.34	2.75
5.0	0.47	3.05
5.5	0.62	3.35
6.0	0.80	3.66
6.5	1.02	3.96
7.0	1.28	4.27
7.5	1.57	4.58
8.0	1.91	4.88
8.5	2.29	5.18
9.0	2.71	5.49
9.5	3.19	5.79
10.0	3.72	6.10
10.5	4.31	6.41
11.0	4.95	6.71
11.5	5.66	7.01
12.0	6.43	7.32

The same size in other stones

A 6.5 mm round in each stone Jart renders. Carat is weight, so a denser stone weighs more at the same size.

Carat weight of a 6.5 mm round in each gemstone
Stone	SG	Carat ct
Diamond	3.52	1.02
Ruby	4.00	1.16
Sapphire	4.00	1.16
Emerald	2.72	0.79
Aquamarine	2.72	0.79
Morganite	2.72	0.79
Amethyst	2.65	0.77
Citrine	2.65	0.77
Quartz	2.65	0.77
Tanzanite	3.35	0.97
Peridot	3.34	0.97
Topaz	3.53	1.02
Garnet	3.90	1.13
Alexandrite	3.73	1.08
Tourmaline	3.06	0.89
Zircon	4.65	1.35
Onyx	2.65	0.77

Common questions

How big is a 1 carat diamond?

A 1 carat round brilliant is about 6.45 mm across and 3.94 mm deep. In other cuts, 1 carat is about 7.88 × 5.84 mm as an oval, 5.56 mm as a princess and 7.22 × 4.81 mm as an emerald cut. On a US 6 finger, 16.5 mm wide, a 1 carat round covers 39% of the width.

What size is a 2 carat diamond?

About 8.13 mm across as a round brilliant. Doubling the weight does not double the size: the length grows with the cube root of the weight, so 2 carats is only 26% wider than 1.

Does a sapphire weigh more than a diamond of the same size?

Yes. Carat is weight, and sapphire's specific gravity is 4.00 against diamond's 3.52, so a 6.5 mm round sapphire is about 1.16 ct where a diamond is 1.02 ct. An emerald of the same size, at 2.72, is about 0.79 ct.

How exact is carat from size?

Within a few percent for a well cut stone. A deeper or shallower cut, a thick girdle or a bulging pavilion change the weight, and only a scale gives the exact figure. In the studio, Design Facts estimates each stone from its own mesh instead of a formula.

How the weight is worked out

The trade estimates a diamond's weight from its measurements: length × width × depth in mm times a factor for the cut. Any other stone weighs that times its specific gravity divided by diamond's.

Round brilliant: diameter² × depth × 0.0061. The other cuts: length × width × depth × oval 0.0062, cushion 0.0080, princess 0.0083, emerald cut 0.0092, pear 0.0060, marquise 0.0058. The converter takes each cut in its usual proportions, length to width and depth as a share of the width: round 1 : 1, 61%; oval 1.35 : 1, 60%; cushion 1.05 : 1, 65%; princess 1 : 1, 70%; emerald cut 1.5 : 1, 65%; pear 1.5 : 1, 60%; marquise 2 : 1, 60%.

On a hand in the studio

Open your ring in Jart and choose On hand: the studio fits the ring to a 3D finger and says whether it is shown at its true size on that finger, and Design Facts lists every stone with its size and estimated carat.

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