Round vs Princess diamond size

By HeyGema™ · Updated November 5, 2026

In short

A 1 carat round measures 6.5 mm and a 1 carat princess 5.5 x 5.5 mm. At the same weight a round faces up about 8% larger than a princess. In face-up terms, 1 carat of round looks like a 1.00 carat round and 1 carat of princess like a 0.88 carat round.

6.5 mm1 ct round
5.5 x 5.5 mm1 ct princess
-8%princess face-up vs round

Carat to mm calculator

ShapeGemCarat

About 5.5 x 5.5 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 princess

5.5 x 5.5 mm

Side by side

1 ct princess inside 1 ct round

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

Round vs Princess diamond size at the same weight

At the same weight a round faces up about 8% larger than a princess. A 1 carat round measures 6.5 mm and a 1 carat princess 5.5 x 5.5 mm. A 1 carat round faces up like a 1.00 carat round, and a 1 carat princess like a 0.88 carat round.

0.5 carat: Round length x width: 5.1 mm; Princess length x width: 4.4 x 4.4 mm; Princess face-up vs round: -8%

0.75 carat: Round length x width: 5.9 mm; Princess length x width: 5.0 x 5.0 mm; Princess face-up vs round: -8%

1 carat: Round length x width: 6.5 mm; Princess length x width: 5.5 x 5.5 mm; Princess face-up vs round: -8%

1.5 carat: Round length x width: 7.4 mm; Princess length x width: 6.3 x 6.3 mm; Princess face-up vs round: -8%

2 carat: Round length x width: 8.1 mm; Princess length x width: 6.9 x 6.9 mm; Princess face-up vs round: -8%

3 carat: Round length x width: 9.3 mm; Princess length x width: 7.9 x 7.9 mm; Princess face-up vs round: -8%

Carat	Round length x width	Princess length x width	Princess face-up vs round
0.5 carat	5.1 mm	4.4 x 4.4 mm	-8%
0.75 carat	5.9 mm	5.0 x 5.0 mm	-8%
1 carat	6.5 mm	5.5 x 5.5 mm	-8%
1.5 carat	7.4 mm	6.3 x 6.3 mm	-8%
2 carat	8.1 mm	6.9 x 6.9 mm	-8%
3 carat	9.3 mm	7.9 x 7.9 mm	-8%

Round vs Princess on a finger

A 1 carat round covers 39% of the width of a size 6 finger and a 1 carat princess covers 33%. Long cuts sit along the finger, so their width is what shows across it.

6.3 mm stone on a 16.5 mm finger: it covers 38% of the widthA 1.5 carat princess on a size 6 finger.

4: Finger width: 14.9 mm; 1.5 ct round covers: 50%; 1.5 ct princess covers: 42%

5: Finger width: 15.7 mm; 1.5 ct round covers: 47%; 1.5 ct princess covers: 40%

6: Finger width: 16.5 mm; 1.5 ct round covers: 45%; 1.5 ct princess covers: 38%

7: Finger width: 17.3 mm; 1.5 ct round covers: 43%; 1.5 ct princess covers: 36%

8: Finger width: 18.1 mm; 1.5 ct round covers: 41%; 1.5 ct princess covers: 35%

US ring size	Finger width	1.5 ct round covers	1.5 ct princess covers
4	14.9 mm	50%	42%
5	15.7 mm	47%	40%
6	16.5 mm	45%	38%
7	17.3 mm	43%	36%
8	18.1 mm	41%	35%

Match the look, not only the carat

Because a princess needs about 1.14 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 Princess size charts

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

Questions

At the same weight a round faces up about 8% larger than a princess. 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 princess about 5.5 x 5.5 mm, for a well-cut stone at the usual ratio.

A 2 carat round is 8.1 mm and a 2 carat princess is 6.9 x 6.9 mm. Face-up, the princess is -8% against the round.

A 1 carat round covers 39% of the width of a size 6 finger and a 1 carat princess 33%. The numbers change with ring size and with the stone's length-to-width ratio.