How big is a 40 carat diamond?
By HeyGema™ · Updated
In short
A 40 carat round diamond measures about 22.1 mm across (0.87 inches) and weighs 8.00 grams. An oval of the same weight is 27.4 x 18.8 mm and an emerald cut 23.9 x 17.1 mm. On a size 6 finger a round 40 carat covers 134% of the width.
- 22.1 mmround diamond, across
- 0.87 inthe same in inches
- 8.00 gweight
- 134%of a size 6 finger's width
Carat to mm calculator
About 22.1 x 22.1 mm
An estimate for a well-cut stone. A deep or shallow cut faces up smaller or bigger.
40 carat diamond size by shape
These are the face-up measurements of a well-cut 40 carat diamond. Weight is the same in every row, but the face-up size is not: the last column compares each cut with a round diamond of the same weight.
- Round
- Length x width: 22.1 mm
- In inches: 0.87 in
- Face-up size vs round: -
- Oval
- Length x width: 27.4 x 18.8 mm
- In inches: 1.08 x 0.74 in
- Face-up size vs round: +6%
- Cushion
- Length x width: 19.2 x 19.2 mm
- In inches: 0.75 x 0.75 in
- Face-up size vs round: -14%
- Princess
- Length x width: 18.8 x 18.8 mm
- In inches: 0.74 x 0.74 in
- Face-up size vs round: -8%
- Emerald cut
- Length x width: 23.9 x 17.1 mm
- In inches: 0.94 x 0.67 in
- Face-up size vs round: +4%
- Asscher
- Length x width: 18.8 x 18.8 mm
- In inches: 0.74 x 0.74 in
- Face-up size vs round: -16%
- Radiant
- Length x width: 22.2 x 17.1 mm
- In inches: 0.88 x 0.67 in
- Face-up size vs round: -2%
- Pear
- Length x width: 29.1 x 18.8 mm
- In inches: 1.14 x 0.74 in
- Face-up size vs round: -3%
- Marquise
- Length x width: 35.6 x 17.8 mm
- In inches: 1.40 x 0.70 in
- Face-up size vs round: +16%
- Heart
- Length x width: 22.2 x 22.2 mm
- In inches: 0.88 x 0.88 in
- Face-up size vs round: -17%
- Trillion
- Length x width: 23.9 x 23.9 mm
- In inches: 0.94 x 0.94 in
- Face-up size vs round: -14%
- Baguette
- Length x width: 27.4 x 13.7 mm
- In inches: 1.08 x 0.54 in
- Face-up size vs round: -2%
40 carat diamond next to a coin
Is a 40 carat diamond big?
It depends on the hand. A round 40 carat covers 148% of the width of a size 4 finger, 134% of a size 6 and 122% of a size 8. At 40 carats a round diamond is 22.1 mm across, about as wide as a US quarter (24.26 mm). A stone this size is usually worn as a pendant or made into a piece for it, not set in a ring.
40 carat diamond on a finger, by ring size
The share of the finger's width that the stone covers, for the four most searched shapes. Elongated stones sit along the finger, so their width is what you see across it. Try other ring sizes in the diamond size on finger tool.
- 4
- Finger width: 14.9 mm
- Round: 148%
- Oval: 126%
- Cushion: 129%
- Emerald cut: 115%
- 5
- Finger width: 15.7 mm
- Round: 141%
- Oval: 120%
- Cushion: 122%
- Emerald cut: 109%
- 6
- Finger width: 16.5 mm
- Round: 134%
- Oval: 114%
- Cushion: 116%
- Emerald cut: 104%
- 7
- Finger width: 17.3 mm
- Round: 127%
- Oval: 109%
- Cushion: 111%
- Emerald cut: 99%
- 8
- Finger width: 18.1 mm
- Round: 122%
- Oval: 104%
- Cushion: 106%
- Emerald cut: 94%
- 9
- Finger width: 18.9 mm
- Round: 116%
- Oval: 99%
- Cushion: 101%
- Emerald cut: 90%
40 carat compared with nearby sizes
Round diamond diameter for the weights on either side of 40 carat. The last column is the difference in width from a 40 carat stone.
- 25 carat
- Round diameter: 18.9 mm
- Difference from 40 ct: -3.20 mm
- 30 carat
- Round diameter: 20.0 mm
- Difference from 40 ct: -2.02 mm
- 40 (this page)
- Round diameter: 22.1 mm
- Difference from 40 ct: -
- 50 carat
- Round diameter: 23.8 mm
- Difference from 40 ct: +1.70 mm
- 75 carat
- Round diameter: 27.2 mm
- Difference from 40 ct: +5.14 mm
Other gems at 40 carat
Gems are not all as dense as diamond, so the same weight faces up bigger or smaller. Round stones of 40 carat:
- Diamond
- Round diameter: 22.1 mm
- Against diamond: -
- Moissanite
- Round diameter: 22.7 mm
- Against diamond: +3% wider
- Sapphire
- Round diameter: 21.1 mm
- Against diamond: -4% wider
- Ruby
- Round diameter: 21.1 mm
- Against diamond: -4% wider
- Emerald
- Round diameter: 24.0 mm
- Against diamond: +9% wider
- Aquamarine
- Round diameter: 24.1 mm
- Against diamond: +9% wider
- Morganite
- Round diameter: 23.8 mm
- Against diamond: +8% wider
- Tanzanite
- Round diameter: 22.4 mm
- Against diamond: +2% wider
- Topaz
- Round diameter: 22.0 mm
- Against diamond: same
- Amethyst
- Round diameter: 24.2 mm
- Against diamond: +10% wider
- Spinel
- Round diameter: 21.9 mm
- Against diamond: -1% wider
How these sizes are worked out
We start from the face-up size of a well-cut 1 carat diamond of each shape and scale it with the cube root of the weight and the gem's density. They are estimates for a typical cut. See the methodology for the formulas, or use the calculator above for any weight.