How big is a 8 carat round diamond?

By HeyGema™ · Updated November 5, 2026

In short

A 8 carat round diamond measures about 12.9 mm. On a size 6 finger it covers 78% of the width. For the same weight, a round diamond is 12.9 mm.

12.9 mm8 carat round
0.51 inthe same in inches
referenceface-up size vs a round
78%of a size 6 finger's width

Carat to mm calculator

ShapeGemCarat

About 12.9 x 12.9 mm

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

8 ct round

12.9 mm

8 ct round

12.9 mm

Side by side

8 ct round inside 8 ct round

A 8 carat round diamond beside a 8 carat round, to the same scale.

8 carat round diamond size

A well-cut 8 carat round diamond measures about 12.9 mm (0.51 inches) at a length-to-width ratio of 1.00:1. It weighs 1.60 grams.

8 carat round on a finger

12.9 mm stone on a 16.5 mm finger: it covers 78% of the widthA 8 carat round diamond on a size 6 finger.

4: Finger width: 14.9 mm; Round covers: 87%

5: Finger width: 15.7 mm; Round covers: 82%

6: Finger width: 16.5 mm; Round covers: 78%

7: Finger width: 17.3 mm; Round covers: 74%

8: Finger width: 18.1 mm; Round covers: 71%

9: Finger width: 18.9 mm; Round covers: 68%

US ring size	Finger width	Round covers
4	14.9 mm	87%
5	15.7 mm	82%
6	16.5 mm	78%
7	17.3 mm	74%
8	18.1 mm	71%
9	18.9 mm	68%

Round sizes near 8 carat

5 carat: Length x width: 11.0 mm; Round of the same weight: 11.0 mm

6 carat: Length x width: 11.7 mm; Round of the same weight: 11.7 mm

8 (this page): Length x width: 12.9 mm; Round of the same weight: 12.9 mm

10 carat: Length x width: 13.9 mm; Round of the same weight: 13.9 mm

Carat	Length x width	Round of the same weight
5 carat	11.0 mm	11.0 mm
6 carat	11.7 mm	11.7 mm
8 (this page)	12.9 mm	12.9 mm
10 carat	13.9 mm	13.9 mm

8 carat in other shapes

Every row is a well-cut diamond of 8 carat. The last column is the face-up size against the round.

Oval: Length x width: 16.0 x 11.0 mm; Against round: +6%

Cushion: Length x width: 11.2 x 11.2 mm; Against round: -14%

Princess: Length x width: 11.0 x 11.0 mm; Against round: -8%

Emerald cut: Length x width: 14.0 x 10.0 mm; Against round: +4%

Asscher: Length x width: 11.0 x 11.0 mm; Against round: -16%

Radiant: Length x width: 13.0 x 10.0 mm; Against round: -2%

Pear: Length x width: 17.0 x 11.0 mm; Against round: -3%

Marquise: Length x width: 20.8 x 10.4 mm; Against round: +16%

Heart: Length x width: 13.0 x 13.0 mm; Against round: -17%

Trillion: Length x width: 14.0 x 14.0 mm; Against round: -14%

Baguette: Length x width: 16.0 x 8.0 mm; Against round: -2%

Shape	Length x width	Against round
Oval	16.0 x 11.0 mm	+6%
Cushion	11.2 x 11.2 mm	-14%
Princess	11.0 x 11.0 mm	-8%
Emerald cut	14.0 x 10.0 mm	+4%
Asscher	11.0 x 11.0 mm	-16%
Radiant	13.0 x 10.0 mm	-2%
Pear	17.0 x 11.0 mm	-3%
Marquise	20.8 x 10.4 mm	+16%
Heart	13.0 x 13.0 mm	-17%
Trillion	14.0 x 14.0 mm	-14%
Baguette	16.0 x 8.0 mm	-2%

Questions

A 8 carat round diamond is about 12.9 mm at the usual 1.00:1 ratio.

A 8 carat round diamond is about 0.51 inches across.

There is no rule. The stone is wider than most fingers. Compare it with a ring you have tried on in the finger chart above.

Yes. A lab grown diamond has the same density as a mined one, so it measures the same for the same weight.