Millimeters vs. Carat Weight Measurements
Understanding the difference between millimeters and carat weight is important for anyone shopping for jewelry. Millimeter (mm) measurements reflect the gemstone’s dimensions, while carats measure weight.
While carat weight influences a diamond’s appearance, it doesn’t directly equate to size. Two diamonds of the same carat weight can look different based on how their weight is distributed. For instance, elongated fancy-shaped diamonds with shallow depths and longer dimensions may appear larger than round diamonds of the same carat weight.
Because other gemstones have unique densities, they can appear larger or smaller than diamonds of the same carat weight. Denser gemstones, like rubies and sapphires, weigh more than diamonds of the same millimeter dimension—for example, a 5 mm ruby will weigh more than a 5 mm diamond.
MM to Carat Conversion Charts
Mm-to-carat conversions are approximate rather than exact. The charts below provide estimated diamond millimeters compared to diamond carat weights. Other gemstones will have different mm to carat weight conversions based on their specific density.
Round and perfectly square gemstones are measured by a single diameter, such as “8 mm,” since they have equal dimensions from all angles. Other shapes are listed with both length and width measurements in millimeters. The length-to-width ratio is a useful guide for creating specific looks—for example, cushion-cut gemstones can appear square or rectangular depending on their ratio. Cushion cuts with a 1:1 ratio appear square, while a 2:1 ratio creates an elongated look. Fancy shapes, like ovals and pears, may appear more rounded or slender depending on their proportions.
For help visualizing diamond or other gemstone sizes, our jewelry specialists can answer your questions and provide imagery. Visit one of our showrooms, book a virtual appointment, or chat live on our website for assistance.
