DIAMOND WEIGHT ESTIMATION FORMULAS
The formulas listed below are often used for estimating the carat weight of a mounted diamond used primarily by jewelers and appraisers of jewelry. Diamond buyers utilize these formulas as well when buying a diamond still set in a mounting.

These calculations do not always give you the exact weight. However they act as a good reference to figure out the carat weight before deciding to removing a diamond.

We recommend a diamond is removed from the mounting or setting and sent to GIA for grading.

A loose diamond can simply be weighed rather than estimating the actual carat weight.

The exact carat weight of your diamond is the utmost importance in pricing your diamond.

Buying your diamond in the ring or mounting requires guesswork. As explained on this site, if a diamond falls below the “magic number” it falls into a different price bracket, thereby lowering the per carat value..

For Example:

Your diamond valuation appraisal estimates the diamond’s carat weight at 3.01 cts based on measurements since it is still set in the ring mounting. This is an approximate weight, not the actual known carat weight measurement by a scale.
Most astute diamond buyers will make their offer based on the lower 2.99ct weight, rather than the 3.01 ct weight as stated on the appraisal. The2.99 carat weight is in a lower price bracket than the 3.0ct or above price bracket.
A diamond buyer will not hope your diamond weighs 3.01ct when it could be 2.99ct. This is too costly of a risk to take. Therefore, expect lower offers for set diamonds that are uncertified as opposed to loose diamonds that have been professionally lab graded.

The only way to truly know the diamond’s carat weight is the have the diamond removed from the setting.

When removing a precious gem, it is best to have a “bench jeweler” perform the removal since they have expertise in setting and removing gemstones.

The following formulas are used in the industry to calculate the approximate value rather than the actual carat weight.

ROUND BRILLIANT
Average Diameter x Diameter x Depth x 0.0061 x Girdle Correction % = Approximate Estimated Carat Weight

PRINCESS CUT
Length x Width x Depth x 0.0082 x Girdle Correction % x Weight Correction% = Approximate Estimated Carat Weight

ASSCHER STEP CUT
Length x Width x Depth x 0.0080 x Girdle Correction % x Weight Correction% = Approximate Estimated Carat Weight

CUSHION BRILLIANT
Length x Width x Depth x 0.00815 x Girdle Correction % x Weight Correction% = Approximate Estimated Carat Weight

HEART BRILLIANT
Length x Width x Depth x 0.0059 x Girdle Correction % x Weight Correction% = Approximate Estimated Carat Weight

TRILLION BRILLIANT
Length x Width x Depth x 0.0057 x Girdle Correction % x Weight Correction% = Approximate Estimated Carat Weight

MARQUISE BRILLIANT
Length x Width x Depth x 0.00565 x GC % x WC% (1½ : 1 ratio)
Length x Width x Depth x 0.0058 x GC % x WC% (2 : 1 ratio)
Length x Width x Depth x 0.00585 x GC% x WC% (2½ : 1 ratio)

PEAR BRILLIANT
Length x Width x Depth x 0.00615 x GC % x WC% (1¼ : 1 ratio)
Length x Width x Depth x 0.0060 x GC % x WC% (1½ : 1 ratio)
Length x Width x Depth x 0.00575 x GC% x WC% (2 : 1 ratio)

OVAL BRILLIANT
Length x Width x Depth x 0.00625 x GC % x WC% (1¼ : 1 ratio)
Length x Width x Depth x 0.0064 x GC % x WC% (1½ : 1 ratio)
Length x Width x Depth x 0.0067 x GC % x WC% (2 : 1 ratio)

EMERALD STEP CUT
Length x Width x Depth x 0.0092 x GC % x WC% (1½ : 1 ratio)
Length x Width x Depth x 0.010 x GC % x WC% (2 : 1 ratio)
Length x Width x Depth x 0.0106 x GC % x WC% (2½ : 1 ratio)

RADIANT BRILLIANT
Length x Width x Depth x 0.0081 x GC % x WC% (1 : 1 ratio)
Length x Width x Depth x 0.0084 x GC % x WC% (1½ : 1 ratio)

Since diamonds are cut from “rough” diamonds (i.e. an uncut or unpolished diamond), most diamonds will have different measurements once finished. The following adjustments can used to calculate final carat weight and adjust for variations in each diamond.

GIRDLE CORRECTION ADJUSTMENTS

Formulas above are for diamonds with THIN – MEDIUM girdles, add % below for thicker girdles
ROUND BRILLIANT (below 1.25cts):
Slightly Thick 2-3% • Thick 3-4% • Very Thick 6-9% • Extremely Thick 8-12%

ROUND BRILLIANT (above 1.25cts):
Slightly Thick 1-2% • Thick 2-3% • Very Thick 4-6% • Extremely Thick 6-8%

FANCY SHAPES
Slightly Thick 3-4% • Thick 5-7% • Very Thick 9-10% • Extremely Thick 11+%

WEIGHT CORRECTION ADJUSTMENTS

Squarish or High Shoulders
Pear +1% to +5%
Oval +1% to +10%

Wide or Fat Wings
Pear +1% to +5%
Marquise +1% to +10%
Large Culet
All Shapes +1% to +2%

Wide Corners
Emerald -1% to -5%
Radiant -1% to -5%
Pavilion Bulge
Emerald, Radiant & Trillion +1% to +15%