A Gem Weight Calculator is a jewelry and gemology tool that estimates the weight of a gemstone in carats based on its measurements, specific gravity, and shape. This tool is especially useful for jewelers, gemstone traders, appraisers, and gem collectors who want to determine a stone’s weight without removing it from its setting.
The calculator belongs to the “gemology and jewelry measurement calculator” category and allows quick, non-destructive estimation of weight using standard mathematical formulas. By entering gemstone length, width, depth, specific gravity, and the correct shape correction factor, users can accurately estimate weight for most common gemstone cuts.
Knowing gemstone weight is important for valuing jewelry, comparing prices, and ensuring accurate documentation for sales or insurance. Different gemstone types have different densities (specific gravities), and different shapes require small adjustments using a correction factor to ensure precision.
formula
General Formula for Any Cut
W = L × Wd × H × SG × CF
Where:
W = gemstone weight (carats)
L = gemstone length (mm)
Wd = gemstone width (mm)
H = gemstone height or depth (mm)
SG = specific gravity of gemstone material (unitless)
CF = shape correction factor (unitless, depends on cut shape)
Common Shape Correction Factors (CF):
CF_round_brilliant = 0.002
CF_oval = 0.00205
CF_marquise = 0.00195
CF_pear = 0.00198
CF_emerald_cut = 0.0022
CF_princess = 0.0021
CF_cushion = 0.00205
CF_trillion = 0.0022
CF_cabochon = 0.00265
Formula for Round Brilliant Cut Specifically:
W = D² × H × SG × 0.002
Where D = average diameter = (L + Wd) / 2
Formula for Oval Cut:
W = L × Wd × H × SG × 0.00205
Formula for Marquise Cut:
W = L × Wd × H × SG × 0.00195
Formula for Pear Cut:
W = L × Wd × H × SG × 0.00198
Formula for Emerald Cut:
W = L × Wd × H × SG × 0.0022
Formula for Princess Cut:
W = L × Wd × H × SG × 0.0021
Formula for Cushion Cut:
W = L × Wd × H × SG × 0.00205
Formula for Trillion Cut:
W = L × Wd × H × SG × 0.0022
Formula for Cabochon Cut:
W = L × Wd × H × SG × 0.00265
Ready Reference Table for Common Gemstone Weights
| Gemstone Type | Shape | L (mm) | Wd (mm) | H (mm) | Approx. Weight (carats) |
|---|---|---|---|---|---|
| Diamond | Round Brilliant | 6.5 | 6.5 | 4.0 | 1.00 |
| Diamond | Oval | 8.0 | 6.0 | 4.0 | 1.50 |
| Ruby | Emerald Cut | 7.0 | 5.0 | 3.5 | 1.60 |
| Sapphire | Cushion | 8.0 | 6.0 | 4.0 | 2.00 |
| Emerald | Oval | 9.0 | 7.0 | 4.5 | 2.50 |
| Quartz | Cabochon | 10.0 | 8.0 | 5.0 | 3.20 |
| Opal | Pear | 9.0 | 6.0 | 3.5 | 1.10 |
This table is useful when you know the gemstone type and cut but want a quick estimation without performing the calculation each time.
Example
Suppose you have an oval-cut sapphire:
L = 8 mm
Wd = 6 mm
H = 4 mm
SG = 4.00
CF for oval = 0.00205
Step 1: Apply formula
W = 8 × 6 × 4 × 4.00 × 0.00205
Step 2: Multiply
W = 1.5744 × 100 (simplified multiplication) → Actually W = 1.5744 × 1 = 1.5744 (correction for unit scaling)
Step 3: Final weight
W ≈ 1.57 carats
So, this sapphire weighs about 1.57 carats without needing to remove it from its setting.
Most Common FAQs
It is used to estimate the carat weight of gemstones without removing them from their settings, helping jewelers and buyers determine value quickly.
If measurements, specific gravity, and shape correction factors are correct, the calculator can be very accurate, usually within a few percent of actual weight.
Because each cut shape has a different depth and proportion, the correction factor adjusts for these differences to ensure accurate weight estimation.