How many carats is a 3 mm diamond?

By HeyGema™ · Updated November 5, 2026

In short

A 3 mm round diamond weighs about 0.1 carat (0.020 grams). A 3 mm princess cut weighs about 0.16 carat, and an oval that is 3 mm long about 0.050 carat.

0.1 ctround diamond, 3 mm
0.020 gweight
0.12 in3 mm in inches

Carat to mm calculator

ShapeGemLength (mm)

About 0.1 carat

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

3 mm diamond weight by shape

The weight of a well-cut diamond measuring 3 mm. For round and square shapes the number is the diameter or the side. For longer shapes it is the long side, with the width following the usual ratio.

Round: Size: 3 mm; Weight: 0.1 ct

Oval: Size: 3 mm x 2.07 mm; Weight: 0.050 ct

Cushion: Size: 3 mm x 3.0 mm; Weight: 0.15 ct

Princess: Size: 3 mm x 3.0 mm; Weight: 0.16 ct

Emerald cut: Size: 3 mm x 2.14 mm; Weight: 0.080 ct

Asscher: Size: 3 mm x 3.0 mm; Weight: 0.16 ct

Radiant: Size: 3 mm x 2.31 mm; Weight: 0.1 ct

Pear: Size: 3 mm x 1.94 mm; Weight: 0.040 ct

Marquise: Size: 3 mm x 1.50 mm; Weight: 0.020 ct

Heart: Size: 3 mm x 3.0 mm; Weight: 0.1 ct

Trillion: Size: 3 mm x 3.0 mm; Weight: 0.080 ct

Baguette: Size: 3 mm x 1.50 mm; Weight: 0.050 ct

Shape	Size	Weight
Round	3 mm	0.1 ct
Oval	3 mm x 2.07 mm	0.050 ct
Cushion	3 mm x 3.0 mm	0.15 ct
Princess	3 mm x 3.0 mm	0.16 ct
Emerald cut	3 mm x 2.14 mm	0.080 ct
Asscher	3 mm x 3.0 mm	0.16 ct
Radiant	3 mm x 2.31 mm	0.1 ct
Pear	3 mm x 1.94 mm	0.040 ct
Marquise	3 mm x 1.50 mm	0.020 ct
Heart	3 mm x 3.0 mm	0.1 ct
Trillion	3 mm x 3.0 mm	0.080 ct
Baguette	3 mm x 1.50 mm	0.050 ct

3 mm stones in other gems

A round stone of the same size weighs differently in each gem, because gems differ in density.

Diamond: Weight of a round stone: 0.1 ct

Moissanite: Weight of a round stone: 0.090 ct

Sapphire: Weight of a round stone: 0.11 ct

Ruby: Weight of a round stone: 0.11 ct

Emerald: Weight of a round stone: 0.080 ct

Aquamarine: Weight of a round stone: 0.080 ct

Topaz: Weight of a round stone: 0.1 ct

Amethyst: Weight of a round stone: 0.080 ct

Spinel: Weight of a round stone: 0.1 ct

Tanzanite: Weight of a round stone: 0.1 ct

Morganite: Weight of a round stone: 0.080 ct

Gem	Weight of a round stone
Diamond	0.1 ct
Moissanite	0.090 ct
Sapphire	0.11 ct
Ruby	0.11 ct
Emerald	0.080 ct
Aquamarine	0.080 ct
Topaz	0.1 ct
Amethyst	0.080 ct
Spinel	0.1 ct
Tanzanite	0.1 ct
Morganite	0.080 ct

3 mm diamond studs

A pair of 3 mm round diamond studs has a total carat weight of about 0.2 carat, 0.1 carat for each stone. See the diamond stud earring size chart.

Nearby sizes

2 mm: Weight: 0.030 ct; Inches: 0.08 in

2.5 mm: Weight: 0.058 ct; Inches: 0.10 in

3 mm (this page): Weight: 0.1 ct; Inches: 0.12 in

3.5 mm: Weight: 0.16 ct; Inches: 0.14 in

4 mm: Weight: 0.24 ct; Inches: 0.16 in

Diameter	Weight	Inches
2 mm	0.030 ct	0.08 in
2.5 mm	0.058 ct	0.10 in
3 mm (this page)	0.1 ct	0.12 in
3.5 mm	0.16 ct	0.14 in
4 mm	0.24 ct	0.16 in

Questions

About 0.1 carat for a well-cut round diamond.

A 3 mm princess cut, measured along one side, weighs about 0.16 carat.

A 3 mm round moissanite is labeled 0.1 carat (diamond equivalent weight) and weighs about 0.090 carat. See the moissanite size chart.

3 mm is 0.12 inches.