Ruby vs diamond size: same carat, different mm
By HeyGema™ · Updated
In short
A round 1 carat ruby measures 6.2 mm and a round 1 carat diamond 6.5 mm, so the ruby is 4% narrower. That comes from density: ruby is 4.00 and diamond is 3.52.
- 6.2 mm1 ct ruby
- 6.5 mm1 ct diamond
- -4%width vs diamond
Carat to mm calculator
About 6.2 x 6.2 mm
An estimate for a well-cut stone. A deep or shallow cut faces up smaller or bigger.
Ruby vs diamond size at the same carat weight
Ruby is denser than diamond (4.00 against 3.52), so a 1 carat ruby 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
- Ruby (round): 4.9 mm
- Difference in width: -4%
- 1 ct
- Diamond (round): 6.5 mm
- Ruby (round): 6.2 mm
- Difference in width: -4%
- 1.5 ct
- Diamond (round): 7.4 mm
- Ruby (round): 7.1 mm
- Difference in width: -4%
- 2 ct
- Diamond (round): 8.1 mm
- Ruby (round): 7.8 mm
- Difference in width: -4%
- 3 ct
- Diamond (round): 9.3 mm
- Ruby (round): 8.9 mm
- Difference in width: -4%
- 4 ct
- Diamond (round): 10.2 mm
- Ruby (round): 9.8 mm
- Difference in width: -4%
- 5 ct
- Diamond (round): 11.0 mm
- Ruby (round): 10.6 mm
- Difference in width: -4%
Ruby vs diamond at the same size in mm
If you shop by millimeters, the weight changes instead. A round ruby of a given size weighs more than a diamond of that size.
- 4 mm
- Diamond weight: 0.24 ct
- Ruby weight: 0.27 ct
- 5 mm
- Diamond weight: 0.47 ct
- Ruby weight: 0.53 ct
- 6 mm
- Diamond weight: 0.8 ct
- Ruby weight: 0.91 ct
- 6.5 mm
- Diamond weight: 1.02 ct
- Ruby weight: 1.16 ct
- 7 mm
- Diamond weight: 1.28 ct
- Ruby weight: 1.45 ct
- 8 mm
- Diamond weight: 1.91 ct
- Ruby weight: 2.17 ct
- 9 mm
- Diamond weight: 2.72 ct
- Ruby weight: 3.09 ct
- 10 mm
- Diamond weight: 3.73 ct
- Ruby weight: 4.23 ct
Ruby 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. Red corundum, the same mineral as sapphire. 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.