Diamond stud earrings size chart: carat total weight to mm

By HeyGema™ · Updated November 5, 2026

In short

Diamond studs are labeled by the weight of the pair (ctw), so each stone is half. A 1 carat total weight pair is two 0.5 carat stones, about 5.1 mm each in round. A 2 ctw pair is about 6.5 mm each.

Carat to mm calculator

ShapeGemCarat

About 5.1 x 5.1 mm

An estimate for a well-cut stone. A deep or shallow cut faces up smaller or bigger.

0.125 ct

3.2 mm

0.25 ct

4.1 mm

0.5 ct

5.1 mm

0.75 ct

5.9 mm

1 ct

6.5 mm

1.5 ct

7.4 mm

One stone of a pair of round studs at 0.25, 0.5, 1, 1.5, 2 and 3 carat total weight, to the same scale.

Stud earring size by total carat weight

Each row is a pair. The weight of one stone is half the total. Sizes are for well-cut stones of each shape; the studs in a pair are matched to within a small difference in size.

0.25 ctw: Each stone: 0.125 ct; Round: 3.2 mm; Princess: 2.75 x 2.75 mm; Cushion: 2.80 x 2.80 mm

0.33 ctw: Each stone: 0.165 ct; Round: 3.5 mm; Princess: 3.0 x 3.0 mm; Cushion: 3.1 x 3.1 mm

0.4 ctw: Each stone: 0.2 ct; Round: 3.8 mm; Princess: 3.2 x 3.2 mm; Cushion: 3.3 x 3.3 mm

0.5 ctw: Each stone: 0.25 ct; Round: 4.1 mm; Princess: 3.5 x 3.5 mm; Cushion: 3.5 x 3.5 mm

0.75 ctw: Each stone: 0.375 ct; Round: 4.7 mm; Princess: 4.0 x 4.0 mm; Cushion: 4.0 x 4.0 mm

1 ctw: Each stone: 0.5 ct; Round: 5.1 mm; Princess: 4.4 x 4.4 mm; Cushion: 4.4 x 4.4 mm

1.25 ctw: Each stone: 0.625 ct; Round: 5.5 mm; Princess: 4.7 x 4.7 mm; Cushion: 4.8 x 4.8 mm

1.5 ctw: Each stone: 0.75 ct; Round: 5.9 mm; Princess: 5.0 x 5.0 mm; Cushion: 5.1 x 5.1 mm

2 ctw: Each stone: 1 ct; Round: 6.5 mm; Princess: 5.5 x 5.5 mm; Cushion: 5.6 x 5.6 mm

2.5 ctw: Each stone: 1.25 ct; Round: 6.9 mm; Princess: 5.9 x 5.9 mm; Cushion: 6.0 x 6.0 mm

3 ctw: Each stone: 1.5 ct; Round: 7.4 mm; Princess: 6.3 x 6.3 mm; Cushion: 6.4 x 6.4 mm

4 ctw: Each stone: 2 ct; Round: 8.1 mm; Princess: 6.9 x 6.9 mm; Cushion: 7.1 x 7.1 mm

5 ctw: Each stone: 2.5 ct; Round: 8.8 mm; Princess: 7.5 x 7.5 mm; Cushion: 7.6 x 7.6 mm

Total weight (pair)	Each stone	Round	Princess	Cushion
0.25 ctw	0.125 ct	3.2 mm	2.75 x 2.75 mm	2.80 x 2.80 mm
0.33 ctw	0.165 ct	3.5 mm	3.0 x 3.0 mm	3.1 x 3.1 mm
0.4 ctw	0.2 ct	3.8 mm	3.2 x 3.2 mm	3.3 x 3.3 mm
0.5 ctw	0.25 ct	4.1 mm	3.5 x 3.5 mm	3.5 x 3.5 mm
0.75 ctw	0.375 ct	4.7 mm	4.0 x 4.0 mm	4.0 x 4.0 mm
1 ctw	0.5 ct	5.1 mm	4.4 x 4.4 mm	4.4 x 4.4 mm
1.25 ctw	0.625 ct	5.5 mm	4.7 x 4.7 mm	4.8 x 4.8 mm
1.5 ctw	0.75 ct	5.9 mm	5.0 x 5.0 mm	5.1 x 5.1 mm
2 ctw	1 ct	6.5 mm	5.5 x 5.5 mm	5.6 x 5.6 mm
2.5 ctw	1.25 ct	6.9 mm	5.9 x 5.9 mm	6.0 x 6.0 mm
3 ctw	1.5 ct	7.4 mm	6.3 x 6.3 mm	6.4 x 6.4 mm
4 ctw	2 ct	8.1 mm	6.9 x 6.9 mm	7.1 x 7.1 mm
5 ctw	2.5 ct	8.8 mm	7.5 x 7.5 mm	7.6 x 7.6 mm

Stud size in mm to carat total weight

If you know the diameter of a round stud, this is the weight of the pair. Measure across the face of the stone, not the setting.

3 mm: Weight of one: 0.1 ct; Total weight of the pair: 0.2 ctw

3.5 mm: Weight of one: 0.16 ct; Total weight of the pair: 0.32 ctw

4 mm: Weight of one: 0.24 ct; Total weight of the pair: 0.48 ctw

4.5 mm: Weight of one: 0.34 ct; Total weight of the pair: 0.68 ctw

5 mm: Weight of one: 0.47 ct; Total weight of the pair: 0.93 ctw

5.5 mm: Weight of one: 0.62 ct; Total weight of the pair: 1.24 ctw

6 mm: Weight of one: 0.8 ct; Total weight of the pair: 1.61 ctw

6.5 mm: Weight of one: 1.02 ct; Total weight of the pair: 2.05 ctw

7 mm: Weight of one: 1.28 ct; Total weight of the pair: 2.56 ctw

8 mm: Weight of one: 1.91 ct; Total weight of the pair: 3.82 ctw

9 mm: Weight of one: 2.72 ct; Total weight of the pair: 5.43 ctw

10 mm: Weight of one: 3.73 ct; Total weight of the pair: 7.45 ctw

Round stone size	Weight of one	Total weight of the pair
3 mm	0.1 ct	0.2 ctw
3.5 mm	0.16 ct	0.32 ctw
4 mm	0.24 ct	0.48 ctw
4.5 mm	0.34 ct	0.68 ctw
5 mm	0.47 ct	0.93 ctw
5.5 mm	0.62 ct	1.24 ctw
6 mm	0.8 ct	1.61 ctw
6.5 mm	1.02 ct	2.05 ctw
7 mm	1.28 ct	2.56 ctw
8 mm	1.91 ct	3.82 ctw
9 mm	2.72 ct	5.43 ctw
10 mm	3.73 ct	7.45 ctw

What ctw means

ctw stands for carat total weight: the weight of all the stones in a piece added together. For studs it is the pair. When a listing says "1 carat studs" it almost always means 1 ctw, two stones of about half a carat, and not two 1 carat stones. If a seller gives the weight of each stone, double it to compare. The diamond size chart gives the size of any single stone.

Questions

A 1 carat total weight pair is two stones of about 0.5 carat, each 5.1 mm in round. A pair of 1 carat stones each (2 ctw) is 6.5 mm each.

A half carat total weight pair has two stones of about 0.25 carat, each 4.1 mm across in round.

Many people choose 0.5 to 1 carat total weight, stones of 4.1 to 5.1 mm. The right size depends on taste and on the size of the ear, so compare the millimeters, not only the weight.

Each stone weighs 1 carat and is 6.5 mm across in round, 5.5 x 5.5 mm in princess and 5.6 x 5.6 mm in cushion.