MM to Carat Conversion
Estimate diamond and gemstone carat weight from millimeter diameter using standard cut shape formulas.
Why Convert Millimeters to Carats?
Jewelers and buyers often know a stone's face-up size in millimeters before they know its weight. Carat weight drives pricing, so a quick mm-to-carat estimate helps compare stones and set expectations before grading.
Standard Gemstone Weight Formula
For faceted gems, weight is estimated from dimensions and a shape factor:
$$ \text{carats} = L \times W \times D \times \text{shape factor} $$Round brilliant cuts often use diameter for both length and width, with depth based on typical proportions. One carat equals 0.2 grams (200 mg).
Shape Factors and Typical Depth
Each cut uses a different density factor. Round brilliants commonly use 0.0061, while princess cuts use about 0.0083 because they are deeper for the same face size. This tool applies typical depth ratios when you enter only diameter.
For full length, width, and depth control, try the Carat Weight Calculator.
Frequently Asked Questions
How accurate is mm to carat conversion?
It is an estimate based on typical proportions. Actual weight varies with cut quality, girdle thickness, and pavilion depth.
What carat weight is a 6 mm round diamond?
Using typical round depth, a 6 mm stone is often near 0.8 carat. Exact weight depends on the specific cut.
Does shape change the conversion?
Yes. Princess, emerald, and radiant cuts use higher shape factors because they are deeper for the same face size.
How many millimeters is two carats?
It varies by shape. A round two-carat stone is often near 8 mm diameter, while other cuts can look larger or smaller face-up.
What is a carat in grams?
One carat equals 0.2 grams. Five carats equals one gram.
Should I use diameter or length and width?
Round stones use diameter. Other shapes are more accurate with separate length and width, which the full carat weight calculator supports.
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