How we work out gem sizes: the method behind every chart
By HeyGema™ · Updated
In short
Every chart starts from the face-up size of a well-cut 1 carat diamond in each shape, scales it with the cube root of the weight, and adjusts for another gem by its density. The result is an estimate for a typical cut, good to a few percent for round stones and less for some fancy shapes.
The idea
A carat measures weight. Weight follows volume, and volume follows the cube of the stone's size, so every side of a stone grows with the cube root of its weight. A 2 carat stone is 26% wider than a 1 carat of the same shape and cut, not twice as wide. Starting from a known 1 carat size, that gives the size at any weight.
Step 1: the 1 carat size of each shape
- Round
- 1 carat length x width: 6.45 mm
- Ratio: 1.00:1
- Outline fills: 79% of its box
- Oval
- 1 carat length x width: 8.00 x 5.50 mm
- Ratio: 1.45:1
- Outline fills: 79% of its box
- Cushion
- 1 carat length x width: 5.60 x 5.60 mm
- Ratio: 1.00:1
- Outline fills: 90% of its box
- Princess
- 1 carat length x width: 5.50 x 5.50 mm
- Ratio: 1.00:1
- Outline fills: 99% of its box
- Emerald cut
- 1 carat length x width: 7.00 x 5.00 mm
- Ratio: 1.40:1
- Outline fills: 97% of its box
- Asscher
- 1 carat length x width: 5.50 x 5.50 mm
- Ratio: 1.00:1
- Outline fills: 91% of its box
- Radiant
- 1 carat length x width: 6.50 x 5.00 mm
- Ratio: 1.30:1
- Outline fills: 98% of its box
- Pear
- 1 carat length x width: 8.50 x 5.50 mm
- Ratio: 1.55:1
- Outline fills: 68% of its box
- Marquise
- 1 carat length x width: 10.40 x 5.20 mm
- Ratio: 2.00:1
- Outline fills: 70% of its box
- Heart
- 1 carat length x width: 6.50 x 6.50 mm
- Ratio: 1.00:1
- Outline fills: 64% of its box
- Trillion
- 1 carat length x width: 7.00 x 7.00 mm
- Ratio: 1.00:1
- Outline fills: 58% of its box
- Baguette
- 1 carat length x width: 8.00 x 4.00 mm
- Ratio: 2.00:1
- Outline fills: 100% of its box
We set these sizes against the sizes printed by large online diamond retailers and a diamond data site. Where a shape differed by more than 0.3 mm from the middle of the published values, we moved to the middle. Ovals, pears, marquise and step cuts differ the most between sources, because each cutter uses its own ratio, and a trillion's sides run from about 6.7 to 7.7 mm at 1 carat.
Step 2: any weight
Length and width at W carat = the 1 carat size x the cube root of W. For another length-to-width ratio we keep the face-up area and change the ratio.
Step 3: any gem
A gem with specific gravity SG has a volume of weight divided by SG. Each side is multiplied by the cube root of (3.52 / SG), where 3.52 is diamond's. That is why a 1 carat sapphire is smaller than a 1 carat diamond and a 1 carat amethyst is larger. Gemstones vary within a range; we use the middle.
- Diamond
- Specific gravity used: 3.52
- Published range: 3.50 to 3.53
- Size against diamond (same weight): 100%
- Moissanite
- Specific gravity used: 3.22
- Published range: 3.20 to 3.22
- Size against diamond (same weight): 103%
- Sapphire
- Specific gravity used: 4.00
- Published range: 3.95 to 4.03
- Size against diamond (same weight): 96%
- Ruby
- Specific gravity used: 4.00
- Published range: 3.95 to 4.03
- Size against diamond (same weight): 96%
- Emerald
- Specific gravity used: 2.72
- Published range: 2.67 to 2.78
- Size against diamond (same weight): 109%
- Aquamarine
- Specific gravity used: 2.70
- Published range: 2.68 to 2.74
- Size against diamond (same weight): 109%
- Morganite
- Specific gravity used: 2.80
- Published range: 2.80 to 2.90
- Size against diamond (same weight): 108%
- Tanzanite
- Specific gravity used: 3.35
- Published range: 3.30 to 3.38
- Size against diamond (same weight): 102%
- Amethyst
- Specific gravity used: 2.65
- Published range: 2.65
- Size against diamond (same weight): 110%
- Citrine
- Specific gravity used: 2.65
- Published range: 2.65
- Size against diamond (same weight): 110%
- Topaz
- Specific gravity used: 3.53
- Published range: 3.49 to 3.57
- Size against diamond (same weight): 100%
- Peridot
- Specific gravity used: 3.34
- Published range: 3.27 to 3.37
- Size against diamond (same weight): 102%
- Garnet
- Specific gravity used: 3.90
- Published range: 3.6 to 4.3
- Size against diamond (same weight): 97%
- Tsavorite
- Specific gravity used: 3.61
- Published range: 3.60 to 3.62
- Size against diamond (same weight): 99%
- Spinel
- Specific gravity used: 3.60
- Published range: 3.58 to 3.61
- Size against diamond (same weight): 99%
- Tourmaline
- Specific gravity used: 3.06
- Published range: 3.00 to 3.10
- Size against diamond (same weight): 105%
- Cubic zirconia
- Specific gravity used: 5.80
- Published range: 5.6 to 6.0
- Size against diamond (same weight): 85%
- Alexandrite
- Specific gravity used: 3.73
- Published range: 3.73
- Size against diamond (same weight): 98%
- Zircon
- Specific gravity used: 4.69
- Published range: 4.60 to 4.70 (high zircon; low zircon is 3.9 to 4.1)
- Size against diamond (same weight): 91%
- Kunzite
- Specific gravity used: 3.18
- Published range: 3.18
- Size against diamond (same weight): 103%
- Iolite
- Specific gravity used: 2.61
- Published range: 2.61
- Size against diamond (same weight): 110%
- Opal
- Specific gravity used: 2.10
- Published range: 1.98 to 2.25
- Size against diamond (same weight): 119%
- Turquoise
- Specific gravity used: 2.76
- Published range: 2.6 to 2.9
- Size against diamond (same weight): 108%
- Jade
- Specific gravity used: 3.34
- Published range: 3.30 to 3.38
- Size against diamond (same weight): 102%
From millimeters to weight
With length, width and depth, a diamond weighs about length x width x depth x a constant for the cut. The constants are in the table on the diamond weight calculator page. For round, pear and heart we checked them against stones listed with measurements and weights; for step cuts we fitted them to listed stones. Without depth we assume the proportions of a well-cut stone and use (size / 1 carat size)^1.5 for the weight.
Fingers
A finger's width at the ring is taken as the inside diameter of a ring of that US size: 11.63 + 0.8128 x size, in mm. A stone's share of the finger is its width across the finger divided by that. A real finger is slightly narrower toward the tip and wider at the knuckle, so the share is a guide.
What the estimate leaves out
- Cut depth and proportions: a deep stone measures smaller across for its weight, a shallow stone larger. The effect is a few percent in diameter.
- Girdle thickness, which adds weight without face-up size.
- Ratio changes with size: large emerald cuts are often cut less elongated than small ones.
- Treatments, inclusions and variety: for example, garnet density runs from about 3.6 to 4.3 across its kinds, and zircon from 3.9 to 4.7.
- The setting, which changes how big a stone looks.
Checking and corrections
The numbers were last reviewed on 5 November 2026 against published retailer charts. If you find a stone that differs from a chart here by more than a few percent, tell us through the contact page with the measurements and we will look at it.