Standard Gem Sizes Chart

Choosing a Standard Setting or a Custom Setting

Under some circumstances, the stone's value makes cutting for maximum yield worthwhile. You can then pay for a custom setting later. However, in other circumstances, the gem's value doesn't justify the cost of a custom setting.

Do Gems Need to Match Standard Settings Precisely?

No, a gem doesn't need to be the precise size of the jewelry setting. A skilled metalsmith can modify a standard setting to accommodate a variety of gem sizes. For example, any round gem can be put in a standard setting. For other shapes, metalsmiths must consider a variety of factors. As a general rule, gems under one carat can fall within 0.1 mm of the setting size. Over one carat, a 0.2 mm variance can usually be accommodated, sometimes more.

Estimating Weights for Gemstones

The carat weights given for standard gem sizes in the following chart apply to diamonds cut to ideal proportions. Typically, gem cutters cut colored gemstones with greater volume than diamonds. Thus, they'll weigh more than the sizes on the chart.

The density or specific gravity (SG) of the material will also affect the weight. For example, a 6.5 mm round diamond, sapphire, and opal, all cut in the same proportions, will all weigh different. The diamond, with an SG of 3.52, will weigh 1 carat. The sapphire, with an SG of 4, will weigh 1.14 carats. The opal, with an SG of 2.15, will only weigh 0.61 carats. (If you know your gemstone density values well, you can estimate the weight of a standard size gem by sight).

There are subtle variations to these values. For example, some charts list a 0.25-carat round diamond as measuring 4.1 mm. Just keep this in mind: don't take these figures too literally. Use this information as a starting point for estimating weights.

Standard Gem Sizes

Rounds		Squares		Ovals
Size	Weight	Size	Weight	Size	Weight
1.6 mm	.02 carat	5 mm	.75 carat	5 x 3 mm	.25 carat
2 mm	.03 carat	6 mm	1.3 carat	6 x 4 mm	.5 carat
2.5 mm	.06 carat	7 mm	2.05 carat	7 x 5 mm	1.0 carat
3 mm	.10 carat	8 mm	2.9 carat	8 x 6 mm	1.5 carat
3.5 mm	.16 carat	9 mm	4.4 carat	9 x 7 mm	2.5 carat
4 mm	.25 carat	10 mm	6.0 carat	10 x 8 mm	3.0 carat
4.5 mm	.33 carat	11 mm	8.0 carat	11 x 9 mm	4.0 carat
5 mm	.50 carat	12 mm	10.35 carat	12 x 10 mm	5.0 carat
5.5 mm	.65 carat	13 mm	13.2 carat	14 x 10 mm	6.0 carat
6 mm	.75 carat	14 mm	16.5 carat	14 x 12 mm	8.0 carat
6.5 mm	1.0 carat	15 mm	20.25 carat	16 x 12 mm	10.5 carat
7 mm	1.25 carat	16 mm	24.6 carat	18 x 13 mm	14.2 carat
8 mm	2.0 carat	20 x 15 mm	20.3 carat
9 mm	2.75 carat	22 x 15 mm	23.5 carat
10 mm	3.5 carat	23 x 17 mm	32.2 carat
11 mm	4.6 carat	25 x 18 mm	43.0 carat
12 mm	6.0 carat	27 x 20 mm	58.5 carat
13 mm	7.75 carat	30 x 22 mm	79.6 carat
14 mm	9.65 carat
15 mm	11.85 carat
16 mm	14.35 carat
17 mm	17.25 carat
18 mm	20.5 carat
19 mm	24.0 carat
20 mm	28.0 carat
21 mm	34.8 carat
22 mm	41.8 carat
23 mm	49.8 carat
24 mm	57.9 carat
25 mm	68.7 carat

Emerald		Pear		Marquis
6 x 4 mm	.5 carat	6 x 5 mm	.7 carat	4 x 2 mm	.10 carat
7 x 5 mm	1.0 carat	7 x 5 mm	.75 carat	5 x 2.5 mm	.25 carat
8 x 6 mm	1.5 carat	8 x 5 mm	1.0 carat	6 x 3 mm	.30 carat
9 x 7 mm	2.5 carat	9 x 6 mm	1.5 carat	8 x 4 mm	.75 carat
10 x 8 mm	3.0 carat	10 x 7 mm	2.0 carat	10 x 5 mm	1.0 carat
11 x 9 mm	4.0 carat	11 x 7 mm	2.25 carat	12 x 6 mm	2.0 carat
12 x 10 mm	5.0 carat	12 x 8 mm	3.0 carat	14 x 7 mm	3.0 carat
14 x 10 mm	6.0 carat	13 x 9 mm	3.5 carat	16 x 8 mm	4.0 carat
14 x 12 mm	8.0 carat	15 x 10 mm	5.5 carat	18 x 9 mm	6.0 carat
16 x 12 mm	10.5 carat	16 x 12 mm	8.6 carat	20 x 10 mm	8.5 carat
18 x 13 mm	14.2 carat	18 x 13 mm	12 carat
20 x 15 mm	20.3 carat	20 x 15 mm	17.3 carat
25 x 18 mm	52.2 carat	22 x 15 mm	18.7 carat
25 x 18 mm	30.6 carat
30 x 22 mm	54.5 carat

For more information on these gem cuts, consult our guide to fancy gem cuts.