How to Estimate Gem Weight in a Jewelry Setting

Step 1: Measure Your Gem

• Measure the gem carefully in every direction.
• Record all your measurements as well as the stone's shape. Take special note of its proportions.
• Finally, look up the specific gravity (SG) for the stone you're examining in our gem listings.

Step 2: Formulas for Estimating Faceted Gem Weight

Apply the proper formula for the gem based on its cut. The following formulas assume you're dealing with well-proportioned gems with a medium/thin girdle. If that's not the case, you'll have to adjust your results. See Step 4 below.

For all cuts except rounds and rectangular cushions, calculate the diameter as the average of the diagonal, length, and width.

Weight estimates are in carats.

Round

Diameter ✕ diameter ✕ depth ✕ SG ✕ 0.0018

Oval

Diameter ✕ diameter ✕ depth ✕ SG ✕ 0.0020

Square

Diameter ✕ diameter ✕ depth ✕ SG ✕ 0.0024

Square Cushion

Diameter ✕ diameter ✕ depth ✕ SG ✕ 0.0018

Square Step Cut

Diameter ✕ diameter ✕ depth ✕ SG ✕ 0.0023

Square with Cut Corners

Diameter ✕ diameter ✕ depth ✕ SG ✕ 0.0024

Rectangle

Length ✕ width ✕ depth ✕ SG ✕ 0.0026

Rectangle Step Cut

Length ✕ width ✕ depth ✕ SG ✕ 0.0025

Rectangle with Cut Corners

Length ✕ width ✕ depth ✕ SG ✕ 0.0026

Rectangular Cushion

Diameter ✕ diameter ✕ depth ✕ SG ✕ 0.0022 (NOTE: diameter = average of length and width)

Pear

Length ✕ width ✕ depth ✕ SG ✕ 0.0018

Heart

Length ✕ width ✕ depth ✕ SG ✕ 0.0021

Marquise

Length ✕ width ✕ depth ✕ SG ✕ 0.0017

Triangle

Length ✕ width ✕ depth ✕ SG ✕ 0.0018

Step 3: Formulas for Estimating Cabochon Gem Weight

Flat-bottomed cabs actually range from 0.0023 to 0.0029. Those with a bottom bulge range from 0.0024 to 0.0030. As a general rule, use the following formulas:

High-Domed Cabs

Length ✕ width ✕ depth ✕ SG ✕ 0.0026

Low-Domed Cabs

Length ✕ width ✕ depth ✕ SG ✕ 0.0029

Step 4: Adjusting Your Gem Weight Estimate

If the stone you're examining isn't well-proportioned or has a very thin or thick girdle, adjust your gem weight estimate as follows:

Girdle

Use an average of the girdle thickness. Don't base your reading on a bulge at the top of a heart or the point of a gem.

• Thin girdle, subtract 1% to 2%.
• Slightly thick, add 1% to 2%.
• Thick, add 3% to 4%.
• Very thick, add 5% to 6%.
• Extra thick, add 7% to 10%.

Pavilion Bulge

• Slight, add 3% to 5%.
• Noticeable, add 6% to 8%.
• Obvious, add 9% to 12%.
• Extreme, add 13% to 18%.

A long culet due to steep pavilion angles can add up to 5%.

Shape Outline

• Wide corners on cut corner squares and rectangles can decrease weight as much as 5%.
• For oval, pear, marquise, and heart cuts, wide wings or high shoulders can add up to 10%. Occasionally, straight shoulders will require a deduction of 1% to 5%.
• Marquise and (sometimes) pears will have a very short keel or none at all. This will reduce the weight by 1% to 3%.
• Triangles with straight sides will require a reduction of up to 10%.