Round vs Radiant diamond size

By HeyGema™ · Updated November 5, 2026

In short

A 1 carat round measures 6.5 mm and a 1 carat radiant 6.5 x 5.0 mm. At the same weight a round faces up about 2% larger than a radiant. In face-up terms, 1 carat of round looks like a 1.00 carat round and 1 carat of radiant like a 0.96 carat round.

6.5 mm1 ct round
6.5 x 5.0 mm1 ct radiant
-2%radiant face-up vs round

Carat to mm calculator

ShapeGemCaratLength to width

About 6.5 x 5.0 mm

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

1 ct round

6.5 mm

1 ct radiant

6.5 x 5.0 mm

Side by side

1 ct radiant inside 1 ct round

A 1 carat round and a 1 carat radiant diamond, to the same scale.

Round vs Radiant diamond size at the same weight

At the same weight a round faces up about 2% larger than a radiant. A 1 carat round measures 6.5 mm and a 1 carat radiant 6.5 x 5.0 mm. A 1 carat round faces up like a 1.00 carat round, and a 1 carat radiant like a 0.96 carat round.

0.5 carat: Round length x width: 5.1 mm; Radiant length x width: 5.2 x 4.0 mm; Radiant face-up vs round: -2%

0.75 carat: Round length x width: 5.9 mm; Radiant length x width: 5.9 x 4.5 mm; Radiant face-up vs round: -2%

1 carat: Round length x width: 6.5 mm; Radiant length x width: 6.5 x 5.0 mm; Radiant face-up vs round: -2%

1.5 carat: Round length x width: 7.4 mm; Radiant length x width: 7.4 x 5.7 mm; Radiant face-up vs round: -2%

2 carat: Round length x width: 8.1 mm; Radiant length x width: 8.2 x 6.3 mm; Radiant face-up vs round: -2%

3 carat: Round length x width: 9.3 mm; Radiant length x width: 9.4 x 7.2 mm; Radiant face-up vs round: -2%

Carat	Round length x width	Radiant length x width	Radiant face-up vs round
0.5 carat	5.1 mm	5.2 x 4.0 mm	-2%
0.75 carat	5.9 mm	5.9 x 4.5 mm	-2%
1 carat	6.5 mm	6.5 x 5.0 mm	-2%
1.5 carat	7.4 mm	7.4 x 5.7 mm	-2%
2 carat	8.1 mm	8.2 x 6.3 mm	-2%
3 carat	9.3 mm	9.4 x 7.2 mm	-2%

Round vs Radiant on a finger

A 1 carat round covers 39% of the width of a size 6 finger and a 1 carat radiant covers 30%. Long cuts sit along the finger, so their width is what shows across it.

5.7 mm stone on a 16.5 mm finger: it covers 35% of the widthA 1.5 carat radiant on a size 6 finger.

4: Finger width: 14.9 mm; 1.5 ct round covers: 50%; 1.5 ct radiant covers: 38%

5: Finger width: 15.7 mm; 1.5 ct round covers: 47%; 1.5 ct radiant covers: 36%

6: Finger width: 16.5 mm; 1.5 ct round covers: 45%; 1.5 ct radiant covers: 35%

7: Finger width: 17.3 mm; 1.5 ct round covers: 43%; 1.5 ct radiant covers: 33%

8: Finger width: 18.1 mm; 1.5 ct round covers: 41%; 1.5 ct radiant covers: 32%

US ring size	Finger width	1.5 ct round covers	1.5 ct radiant covers
4	14.9 mm	50%	38%
5	15.7 mm	47%	36%
6	16.5 mm	45%	35%
7	17.3 mm	43%	33%
8	18.1 mm	41%	32%

Match the look, not only the carat

Because a radiant needs about 1.04 carat to face up like 1 carat of round, comparing by weight alone hides the difference. Compare millimeters, or put both stones side by side in the size comparison tool.

Round and Radiant size charts

Full charts from 0.25 to 5 carat: Round diamond size chart and Radiant diamond size chart.

Questions

At the same weight a round faces up about 2% larger than a radiant. Face-up area is what you see from above; the round has more of it at equal weight.

A 1 carat round is about 6.5 mm and a 1 carat radiant about 6.5 x 5.0 mm, for a well-cut stone at the usual ratio.

A 2 carat round is 8.1 mm and a 2 carat radiant is 8.2 x 6.3 mm. Face-up, the radiant is -2% against the round.

A 1 carat round covers 39% of the width of a size 6 finger and a 1 carat radiant 30%. The numbers change with ring size and with the stone's length-to-width ratio.