About the Gemstone Carat Weight Calculator
This gemstone carat weight calculator estimates how much a faceted stone weighs from three measurements — length, width and depth in millimetres — together with its shape and its specific gravity (how dense the material is). It is the same estimation method gemologists use when a stone is set in jewelry and cannot be put on a scale.
Use it to sanity-check a listing that gives only millimetre dimensions, to compare how big a 1-carat emerald looks next to a 1-carat sapphire, or to estimate the weight of stones in an heirloom ring. Denser gems weigh more at the same size: a ruby is about 47% heavier than an emerald of identical dimensions.
Results are estimates. The shape factors assume a well-proportioned commercial cut; stones with bulging pavilions, thick girdles or shallow cuts can weigh noticeably more or less. For a definitive weight, a loose stone must be weighed on a calibrated carat scale.
With the default inputs, the estimated weight is 1.54 ct. Change any value above to recalculate instantly.
How to use the gemstone carat weight calculator
- 1Choose the gemstone, or select Other and enter its specific gravity.
- 2Pick the cut shape.
- 3Measure length, width and depth in millimetres with a gauge or calipers.
- 4Read the estimated carat weight and the gram equivalent.
- 5Compare the chart to see how the same size weighs in other gems.
Formula and method
A faceted stone’s weight is its volume times its density. Volume is approximated by multiplying the length, width and depth in millimetres and applying a shape factor F that accounts for the faceted outline and pavilion taper. Multiplying by specific gravity (density in g/cm³) and the factor converts the result directly to carats, since 1 carat is 0.2 g.
The factors used are the standard colored-stone estimation constants: round 0.0018, oval 0.0020, emerald-cut rectangle 0.0025, square cuts 0.0023, pear 0.00175 and marquise 0.0016. For round stones the two diameters are averaged and squared. These constants assume average commercial proportions; a deep or bulky pavilion adds weight that the formula cannot see.
- L, W, D
- Length, width and total depth in millimetres
- SG
- Specific gravity of the gem material
- F
- Shape factor for the cut
Worked examples
8 × 6 × 4 mm oval sapphire
8 × 6 × 4 = 192, times the oval factor 0.0020 is 0.384, and times sapphire’s specific gravity of 4.00 gives about 1.54 carats (0.31 g).
6.5 mm round diamond, 3.9 mm deep
6.5² × 3.9 = 164.8, times 0.0018 times 3.52 gives about 1.04 carats — in line with the familiar rule that a well-cut 1-carat round diamond is roughly 6.4–6.5 mm across.
9 × 7 × 5 mm emerald-cut emerald
9 × 7 × 5 = 315, times the emerald-cut factor 0.0025 and emerald’s specific gravity of 2.72 gives about 2.14 carats.
Frequently asked questions
How accurate is estimating carat weight from measurements?+
For well-proportioned stones the estimate is usually close, but deep pavilions, thick girdles or unusual cuts can push the real weight noticeably above or below it. Only a calibrated scale on a loose stone gives an exact weight.
Why does a ruby weigh more than an emerald of the same size?+
Weight depends on density. Corundum (ruby and sapphire) has a specific gravity of about 4.00, while beryl (emerald) is about 2.72, so a ruby weighs roughly 47% more than an emerald with identical dimensions.
How many grams is a carat?+
One metric carat is exactly 0.2 grams (200 milligrams), so five carats make one gram. Carat weight is different from gold karat, which measures purity.
How do I measure a stone that is set in jewelry?+
Use a millimetre gauge or digital calipers for length and width across the girdle. Depth is harder when the stone is set; measure from the table to the culet if accessible, or estimate it from the setting height.
What specific gravity should I use for a gem not in the list?+
Look up the mineral’s typical specific gravity in a gemology reference and choose Other. Many gems vary slightly by composition, so use the middle of the published range.