How many carats is a 18 mm diamond?
By HeyGema™ · Updated
In short
A 18 mm round diamond weighs about 21.73 carat (4.35 grams). A 18 mm princess cut weighs about 35.05 carat, and an oval that is 18 mm long about 11.44 carat.
- 21.73 ctround diamond, 18 mm
- 4.35 gweight
- 0.71 in18 mm in inches
Carat to mm calculator
About 21.73 carat
An estimate for a well-cut stone. A deep or shallow cut faces up smaller or bigger.
18 mm diamond weight by shape
The weight of a well-cut diamond measuring 18 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: 18 mm
- Weight: 21.73 ct
- Oval
- Size: 18 mm x 12.4 mm
- Weight: 11.44 ct
- Cushion
- Size: 18 mm x 18.0 mm
- Weight: 33.21 ct
- Princess
- Size: 18 mm x 18.0 mm
- Weight: 35.05 ct
- Emerald cut
- Size: 18 mm x 12.9 mm
- Weight: 17 ct
- Asscher
- Size: 18 mm x 18.0 mm
- Weight: 35.05 ct
- Radiant
- Size: 18 mm x 13.8 mm
- Weight: 21.24 ct
- Pear
- Size: 18 mm x 11.6 mm
- Weight: 9.45 ct
- Marquise
- Size: 18 mm x 9.0 mm
- Weight: 5.18 ct
- Heart
- Size: 18 mm x 18.0 mm
- Weight: 21.24 ct
- Trillion
- Size: 18 mm x 18.0 mm
- Weight: 17 ct
- Baguette
- Size: 18 mm x 9.0 mm
- Weight: 11.39 ct
18 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: 21.73 ct
- Moissanite
- Weight of a round stone: 19.88 ct
- Sapphire
- Weight of a round stone: 24.7 ct
- Ruby
- Weight of a round stone: 24.7 ct
- Emerald
- Weight of a round stone: 16.79 ct
- Aquamarine
- Weight of a round stone: 16.67 ct
- Topaz
- Weight of a round stone: 21.8 ct
- Amethyst
- Weight of a round stone: 16.36 ct
- Spinel
- Weight of a round stone: 22.23 ct
- Tanzanite
- Weight of a round stone: 20.68 ct
- Morganite
- Weight of a round stone: 17.29 ct
18 mm diamond studs
A pair of 18 mm round diamond studs has a total carat weight of about 43.47 carat, 21.73 carat for each stone. See the diamond stud earring size chart.
Nearby sizes
Questions
About 21.73 carat for a well-cut round diamond.
A 18 mm princess cut, measured along one side, weighs about 35.05 carat.
A 18 mm round moissanite is labeled 21.73 carat (diamond equivalent weight) and weighs about 19.88 carat. See the moissanite size chart.
18 mm is 0.71 inches.