How big is a 1.25 carat radiant cut diamond?

By HeyGema™ · Updated November 5, 2026

In short

A 1.25 carat radiant cut diamond measures about 7.0 x 5.4 mm. It faces up 2% smaller than a round diamond of the same weight. On a size 6 finger it covers 33% of the width. For the same weight, a round diamond is 6.9 mm.

7.0 x 5.4 mm1.25 carat radiant
0.28 x 0.21 inthe same in inches
-2%face-up size vs a round
33%of a size 6 finger's width

Carat to mm calculator

ShapeGemCaratLength to width

About 7.0 x 5.4 mm

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

1.25 ct radiant

7.0 x 5.4 mm

1.25 ct round

6.9 mm

Side by side

1.25 ct radiant inside 1.25 ct round

A 1.25 carat radiant diamond beside a 1.25 carat round, to the same scale.

1.25 carat radiant diamond size

A well-cut 1.25 carat radiant cut diamond measures about 7.0 x 5.4 mm (0.28 x 0.21 inches) at a length-to-width ratio of 1.30:1. It faces up 2% smaller than a round diamond of the same weight. It weighs 0.25 grams.

1.25 carat radiant size by length-to-width ratio

The same weight can be long and narrow or short and wide. These are 1.25 carat sizes at each ratio. A longer ratio runs the stone along the finger and makes it look slimmer. The face-up area stays almost the same.

1.00:1: Length x width: 6.1 x 6.1 mm; Width on the finger: 37% of a size 6 finger

1.15:1: Length x width: 6.6 x 5.7 mm; Width on the finger: 35% of a size 6 finger

1.30:1: Length x width: 7.0 x 5.4 mm; Width on the finger: 33% of a size 6 finger

1.40:1: Length x width: 7.3 x 5.2 mm; Width on the finger: 31% of a size 6 finger

Ratio	Length x width	Width on the finger
1.00:1	6.1 x 6.1 mm	37% of a size 6 finger
1.15:1	6.6 x 5.7 mm	35% of a size 6 finger
1.30:1	7.0 x 5.4 mm	33% of a size 6 finger
1.40:1	7.3 x 5.2 mm	31% of a size 6 finger

1.25 carat radiant on a finger

5.4 mm stone on a 16.5 mm finger: it covers 33% of the widthA 1.25 carat radiant diamond on a size 6 finger.

4: Finger width: 14.9 mm; Radiant covers: 36%

5: Finger width: 15.7 mm; Radiant covers: 34%

6: Finger width: 16.5 mm; Radiant covers: 33%

7: Finger width: 17.3 mm; Radiant covers: 31%

8: Finger width: 18.1 mm; Radiant covers: 30%

9: Finger width: 18.9 mm; Radiant covers: 28%

US ring size	Finger width	Radiant covers
4	14.9 mm	36%
5	15.7 mm	34%
6	16.5 mm	33%
7	17.3 mm	31%
8	18.1 mm	30%
9	18.9 mm	28%

Radiant sizes near 1.25 carat

0.75 carat: Length x width: 5.9 x 4.5 mm; Round of the same weight: 5.9 mm

1 carat: Length x width: 6.5 x 5.0 mm; Round of the same weight: 6.5 mm

1.25 (this page): Length x width: 7.0 x 5.4 mm; Round of the same weight: 6.9 mm

1.5 carat: Length x width: 7.4 x 5.7 mm; Round of the same weight: 7.4 mm

1.75 carat: Length x width: 7.8 x 6.0 mm; Round of the same weight: 7.8 mm

Carat	Length x width	Round of the same weight
0.75 carat	5.9 x 4.5 mm	5.9 mm
1 carat	6.5 x 5.0 mm	6.5 mm
1.25 (this page)	7.0 x 5.4 mm	6.9 mm
1.5 carat	7.4 x 5.7 mm	7.4 mm
1.75 carat	7.8 x 6.0 mm	7.8 mm

1.25 carat in other shapes

Every row is a well-cut diamond of 1.25 carat. The last column is the face-up size against the radiant.

Round: Length x width: 6.9 mm; Against radiant: +2%

Oval: Length x width: 8.6 x 5.9 mm; Against radiant: +8%

Cushion: Length x width: 6.0 x 6.0 mm; Against radiant: -12%

Princess: Length x width: 5.9 x 5.9 mm; Against radiant: -6%

Emerald cut: Length x width: 7.5 x 5.4 mm; Against radiant: +6%

Asscher: Length x width: 5.9 x 5.9 mm; Against radiant: -13%

Pear: Length x width: 9.2 x 5.9 mm; Against radiant: same

Marquise: Length x width: 11.2 x 5.6 mm; Against radiant: +19%

Heart: Length x width: 7.0 x 7.0 mm; Against radiant: -15%

Trillion: Length x width: 7.5 x 7.5 mm; Against radiant: -11%

Baguette: Length x width: 8.6 x 4.3 mm; Against radiant: same

Shape	Length x width	Against radiant
Round	6.9 mm	+2%
Oval	8.6 x 5.9 mm	+8%
Cushion	6.0 x 6.0 mm	-12%
Princess	5.9 x 5.9 mm	-6%
Emerald cut	7.5 x 5.4 mm	+6%
Asscher	5.9 x 5.9 mm	-13%
Pear	9.2 x 5.9 mm	same
Marquise	11.2 x 5.6 mm	+19%
Heart	7.0 x 7.0 mm	-15%
Trillion	7.5 x 7.5 mm	-11%
Baguette	8.6 x 4.3 mm	same

Questions

A 1.25 carat radiant cut diamond is about 7.0 x 5.4 mm at the usual 1.30:1 ratio.

No. Face-up, a 1.25 carat radiant is 2% smaller than a 1.25 carat round (6.9 mm).

There is no rule. The stone is 36% of the width of a size 4 finger and 30% of a size 8. 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.