Standard Gem Sizes Chart


emerald cut tourmaline - standard gem sizes
Although named after the emerald, which often receives this rectangular cut with trimmed corners, the popular emerald cut design can be applied to many gemstones. It has a range of standard gem sizes for mass-produced jewelry settings. “Emerald Cut Tourmaline,” 4.52 cts, slight color shift. © All That Glitters. Used with permission.

Choosing a Standard Setting or a Custom Setting

Mass-produced jewelry settings come in standard gem sizes. If a gem won’’t fit in a standard setting, you should have a setting custom made. This involves much more labor and, therefore, expense. When purchasing a gem or selecting a cut design, bear this in mind. 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 Gem Cutting Terms.