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.
Round MM to Carat Conversion Chart
Round is the most popular shape, with straightforward conversions due to their uniform diameter.
| Approximate MM Diamond Size | Approximate Diamond Carat Weight |
|---|---|
| 4mm | 0.25ct |
| 5mm | 0.5ct |
| 6.3mm | 1ct |
| 7mm | 1.25ct |
| 7.3mm | 1.5ct |
| 8mm | 2ct |
| 8.7mm | 2.5ct |
| 9.3mm | 3ct |
| 9.7mm | 3.5ct |
| 10.2mm | 4ct |
| 10.6mm | 4.5ct |
| 11mm | 5ct |
Oval MM to Carat Conversion Chart
Ovals are elongated and typically appear larger than round shapes of the same carat weight.
| Approximate MM Diamond Size | Approximate Diamond Carat Weight |
|---|---|
| 6.5×4.5mm | 0.5 |
| 8×5.5mm | 1 |
| 8.8×6mm | 1.25 |
| 9×6.5mm | 1.5 |
| 10×7mm | 2 |
| 11×7.5mm | 2.5 |
| 11.5×8mm | 3 |
| 12.5×8.5mm | 3.5 |
| 13×9mm | 4 |
| 13.5×7mm | 4.5 |
| 14×9.5mm | 5 |
Emerald Cut MM to Carat Conversion Chart
Emerald cuts, known for their step-cut facets, appear slightly larger than round shapes of similar carat weight due to their rectangular shape.
| Approximate MM Diamond Size | Approximate Diamond Carat Weight |
|---|---|
| 5.5×4mm | 0.5 |
| 7×4.5mm | 1 |
| 7.5×5mm | 1.25 |
| 8×5.5mm | 1.5 |
| 8.5×6mm | 2 |
| 9×6.5mm | 2.5 |
| 9.7×7mm | 3 |
| 10.3×7mm | 3.5 |
| 10.7×7.5mm | 4 |
| 11×6mm | 4.5 |
| 11.5×8mm | 5 |
Cushion MM to Carat Conversion Chart
Cushion cuts feature rounded edges and a softer aesthetic, ideal for those seeking a blend of round and square shapes.
| Approximate MM Diamond Size | Approximate Diamond Carat Weight |
|---|---|
| 4.5mm | 0.5 |
| 5.7mm | 1 |
| 6.2mm | 1.25 |
| 6.5mm | 1.5 |
| 7.3mm | 2 |
| 8mm | 2.5 |
| 8.3mm | 3 |
| 8.7mm | 3.5 |
| 9mm | 4 |
| 9.5mm | 4.5 |
| 10mm | 5 |
Pear Shaped MM to Carat Conversion Chart
Combining round and marquise shapes, pear-cuts appear larger than their round counterparts due to their length.
| Approximate MM Diamond Size | Approximate Diamond Carat Weight |
|---|---|
| 7×4.5mm | 0.5 |
| 9×5.5mm | 1 |
| 9.5×6mm | 1.25 |
| 10×6.5mm | 1.5 |
| 11×7mm | 2 |
| 12×7.5mm | 2.5 |
| 13×8mm | 3 |
| 13.5×8.5mm | 3.5 |
| 14.5×9mm | 4 |
| 15×7mm | 4.5 |
| 15.5×9.5mm | 5 |
Radiant MM to Carat Conversion Chart
Radiant cuts have a square or rectangular shape with a brilliant-cut faceting.
| Approximate MM Diamond Size | Approximate Diamond Carat Weight |
|---|---|
| 4.3mm | 0.5 |
| 5.5mm | 1 |
| 6mm | 1.25 |
| 6.5mm | 1.5 |
| 7mm | 2 |
| 7.5mm | 2.5 |
| 8mm | 3 |
| 8.5mm | 3.5 |
| 8.8mm | 4 |
| 9mm | 4.5 |
| 9.5mm | 5 |
Princess MM to Carat Conversion Chart
Princess cuts are square with excellent brilliance, but they tend to appear slightly smaller than round cuts of the same carat weight.
| Approximate MM Diamond Size | Approximate Diamond Carat Weight |
|---|---|
| 4.4mm | 0.5 |
| 5.5mm | 1 |
| 5.9mm | 1.25 |
| 6.3mm | 1.5 |
| 7mm | 2 |
| 7.5mm | 2.5 |
| 8mm | 3 |
| 8.3mm | 3.5 |
| 8.7mm | 4 |
| 9mm | 4.5 |
| 9.3mm | 5 |
Marquise MM to Carat Conversion Chart
Marquise is an elongated shape, which maximizes surface area and tends to look larger than other cuts.
| Approximate MM Diamond Size | Approximate Diamond Carat Weight |
|---|---|
| 8×4mm | 0.5 |
| 10.5×5mm | 1 |
| 11.5×7mm | 1.25 |
| 11.7×6mm | 1.5 |
| 13×6.5mm | 2 |
| 14×7mm | 2.5 |
| 15×7.5mm | 3 |
| 16×8mm | 3.5 |
| 16.5×8.3mm | 4 |
| 17.5×6.5mm | 4.5 |
| 17.7×9mm | 5 |
Asscher MM to Carat Conversion Chart
Asscher cuts have a unique square shape with deep step-cut facets.
| Approximate MM Diamond Size | Approximate Diamond Carat Weight |
|---|---|
| 4.4mm | 0.5 |
| 5.5mm | 1 |
| 6mm | 1.25 |
| 6.3mm | 1.5 |
| 7mm | 2 |
| 7.5mm | 2.5 |
| 8mm | 3 |
| 8.3mm | 3.5 |
| 8.6mm | 4 |
| 9mm | 4.5 |
| 9.7mm | 5 |
Heart Shaped MM to Carat Conversion Chart
Heart shapes, a modification of the pear shape, are measured at their widest point.
| Approximate MM Diamond Size | Approximate Diamond Carat Weight |
|---|---|
| 5.5×5mm | 0.5 |
| 7×6mm | 1 |
| 7.6×6.5mm | 1.25 |
| 8×7mm | 1.5 |
| 9×8mm | 2 |
| 9.5×8.5mm | 2.5 |
| 10×9mm | 3 |
| 10.5×9.5mm | 3.5 |
| 11×10mm | 4 |
| 11.5×7.5mm | 4.5 |
| 12×10.5mm | 5 |
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