Sapphire vs diamond size: same carat, different mm

By HeyGema™ · Updated November 5, 2026

In short

A round 1 carat sapphire measures 6.2 mm and a round 1 carat diamond 6.5 mm, so the sapphire is 4% narrower. That comes from density: sapphire is 4.00 and diamond is 3.52.

6.2 mm1 ct sapphire
6.5 mm1 ct diamond
-4%width vs diamond

Carat to mm calculator

ShapeGemCarat

About 6.2 x 6.2 mm

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

2 ct diamond

8.1 mm

2 ct sapphire

7.8 mm

Side by side

2 ct sapphire inside 2 ct diamond

A 2 carat diamond and a 2 carat sapphire, round, to the same scale.

Sapphire vs diamond size at the same carat weight

Sapphire is denser than diamond (4.00 against 3.52), so a 1 carat sapphire measures 6.2 mm against 6.5 mm for a diamond. A denser stone packs the same weight into less volume, so it is smaller.

0.5 ct: Diamond (round): 5.1 mm; Sapphire (round): 4.9 mm; Difference in width: -4%

1 ct: Diamond (round): 6.5 mm; Sapphire (round): 6.2 mm; Difference in width: -4%

1.5 ct: Diamond (round): 7.4 mm; Sapphire (round): 7.1 mm; Difference in width: -4%

2 ct: Diamond (round): 8.1 mm; Sapphire (round): 7.8 mm; Difference in width: -4%

3 ct: Diamond (round): 9.3 mm; Sapphire (round): 8.9 mm; Difference in width: -4%

4 ct: Diamond (round): 10.2 mm; Sapphire (round): 9.8 mm; Difference in width: -4%

5 ct: Diamond (round): 11.0 mm; Sapphire (round): 10.6 mm; Difference in width: -4%

Carat weight	Diamond (round)	Sapphire (round)	Difference in width
0.5 ct	5.1 mm	4.9 mm	-4%
1 ct	6.5 mm	6.2 mm	-4%
1.5 ct	7.4 mm	7.1 mm	-4%
2 ct	8.1 mm	7.8 mm	-4%
3 ct	9.3 mm	8.9 mm	-4%
4 ct	10.2 mm	9.8 mm	-4%
5 ct	11.0 mm	10.6 mm	-4%

Sapphire vs diamond at the same size in mm

If you shop by millimeters, the weight changes instead. A round sapphire of a given size weighs more than a diamond of that size.

4 mm: Diamond weight: 0.24 ct; Sapphire weight: 0.27 ct

5 mm: Diamond weight: 0.47 ct; Sapphire weight: 0.53 ct

6 mm: Diamond weight: 0.8 ct; Sapphire weight: 0.91 ct

6.5 mm: Diamond weight: 1.02 ct; Sapphire weight: 1.16 ct

7 mm: Diamond weight: 1.28 ct; Sapphire weight: 1.45 ct

8 mm: Diamond weight: 1.91 ct; Sapphire weight: 2.17 ct

9 mm: Diamond weight: 2.72 ct; Sapphire weight: 3.09 ct

10 mm: Diamond weight: 3.73 ct; Sapphire weight: 4.23 ct

Round size	Diamond weight	Sapphire weight
4 mm	0.24 ct	0.27 ct
5 mm	0.47 ct	0.53 ct
6 mm	0.8 ct	0.91 ct
6.5 mm	1.02 ct	1.16 ct
7 mm	1.28 ct	1.45 ct
8 mm	1.91 ct	2.17 ct
9 mm	2.72 ct	3.09 ct
10 mm	3.73 ct	4.23 ct

Sapphire facts that affect size

Specific gravity 3.95 to 4.03, hardness 9 on the Mohs scale and refractive index 1.762 to 1.770. Corundum, the same mineral as ruby. Denser than diamond, so it faces up smaller for its weight.

How much the cut changes this

These sizes assume a well-cut stone of typical depth. A deep cut hides weight in the lower part of the stone and measures smaller across; a shallow cut spreads it and measures larger. Treat the figures as a guide within a few percent. See the methodology.

Questions

No. A 1 carat sapphire measures 6.2 mm against 6.5 mm for a diamond because sapphire has a higher density.

A round 2 carat sapphire is about 7.8 mm. A 2 carat diamond is 8.1 mm.

A round sapphire of 6.5 mm weighs about 1.14 carat.

The density ratio is the same in every shape, so every side of a sapphire is 4% smaller than a diamond of the same carat weight and shape. Use the calculator for other shapes.