Gemstone Carat Weight Calculator

Gemologists use the formula: Length x Width x Depth (in mm) x Specific Gravity x Shape Factor. This gives the "estimated weight" for a mounted stone that cannot be removed for weighing. The specific gravity (density) varies by gem type: diamonds are 3.52, sapphires and rubies are 4.00, and emeralds are 2.72. Shape factors calibrate the formula for different cut profiles: round stones use 0.0018, ovals use 0.0020, and emerald cuts use 0.0025.

These are professional appraiser corrections that improve accuracy beyond the basic formula. The girdle is the outer rim of the stone. A thick girdle adds hidden weight (2 to 10% extra) that the basic dimensions do not capture. The pavilion is the bottom cone of the gem. Colored stones like sapphires and rubies are often cut with a deep, rounded belly to enhance color saturation, adding 5 to 10% more weight. Without these adjustments, the basic formula can underestimate carat weight by 5 to 15%.

The standard L x W x D x SG x SF formula is accurate to within 5 to 10% for well-proportioned stones. Adding girdle and pavilion corrections narrows this to 2 to 5% accuracy. A calibrated jeweler's scale (accurate to 0.01 ct) is always more precise, but this calculator is invaluable for: estimating mounted stones that cannot be removed, verifying a seller's carat claim before purchase, and appraising loose stones when a scale is unavailable.

Shape factors compensate for the volume of material removed during cutting. A round brilliant has the most material removed (shape factor 0.0018) because the circular outline wastes more rough stone. An emerald cut retains more material (shape factor 0.0025) due to its rectangular profile. A marquise is the thinnest (0.0016) because its pointed ends contain very little material. These factors are empirically calibrated from thousands of measured stones.

Sapphires and rubies (corundum) have a specific gravity of 4.00, while diamonds have 3.52. This means a sapphire of the same dimensions as a diamond weighs approximately 14% more. A 7mm round diamond weighs about 1.25 ct, while a 7mm round sapphire weighs about 1.70 ct. Zircon (SG 4.70) is even denser. This is why you should never estimate a colored stone's weight using diamond weight charts.

Depth percentage is the depth divided by the average of length and width, expressed as a percentage. For round diamonds, ideal depth is 59 to 62.5%. Shallower stones (under 57%) look larger face-up but may lack brilliance. Deeper stones (over 65%) look smaller face-up but weigh more. Colored stones often have deeper cuts (65 to 75%) to maximize color saturation. Depth percentage helps identify whether a stone is well-proportioned or carrying hidden weight in its depth.

Yes, this is one of the primary use cases. For mounted stones, measure the visible dimensions (length and width) using a millimeter gauge or digital caliper. Estimate the depth based on the setting style: prong settings allow depth measurement from the side, while bezel settings require estimation. For bezel-set stones, assume depth is 60 to 65% of the average of length and width as a starting point. The girdle and bulge adjustments help compensate for estimation uncertainty.