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Assortment of loose diamonds in different carat weights and shapes.Assortment of loose diamonds in different carat weights and shapes.

MM to Carat Guide

Learn how size and weight differ with our millimeter-to-carat weight guide for diamonds.

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.

Loose Diamonds and Carat Weight Stand

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

Round is the most popular shape, with straightforward conversions due to their uniform diameter.


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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

Ovals are elongated and typically appear larger than round shapes of the same carat weight.


Shop Oval Natural Diamonds | Shop Oval Lab Diamonds

Approximate MM Diamond Size

Approximate Diamond Carat Weight

6.5x4.5mm

0.5

8x5.5mm

1

8.8x6mm

1.25

9x6.5mm

1.5

10x7mm

2

11x7.5mm

2.5

11.5x8mm

3

12.5x8.5mm

3.5

13x9mm

4

13.5x7mm

4.5

14x9.5mm

5

Emerald MM to Carat Conversion

Emerald cuts, known for their step-cut facets, appear slightly larger than round shapes of similar carat weight due to their rectangular shape.


Shop Emerald Cut Natural Diamonds | Shop Emerald Cut Lab Diamonds

Approximate MM Diamond Size

Approximate Diamond Carat Weight

5.5x4mm

0.5

7x4.5mm

1

7.5x5mm

1.25

8x5.5mm

1.5

8.5x6mm

2

9x6.5mm

2.5

9.7x7mm

3

10.3x7mm

3.5

10.7x7.5mm

4

11x6mm

4.5

11.5x8mm

5

Cushion MM to Carat Conversion

Cushion cuts feature rounded edges and a softer aesthetic, ideal for those seeking a blend of round and square shapes.


Shop Cushion Natural Diamonds | Shop Cushion Lab Diamonds

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 MM to Carat Conversion

Combining round and marquise shapes, pear-cuts appear larger than their round counterparts due to their length.


Shop Pear Natural Diamonds | Shop Pear Lab Diamonds

Approximate MM Diamond Size

Approximate Diamond Carat Weight

7x4.5mm 

0.5

9x5.5mm

1

9.5x6mm

1.25

10x6.5mm

1.5

11x7mm

2

12x7.5mm

2.5

13x8mm

3

13.5x8.5mm

3.5

14.5x9mm 

4

15x7mm

4.5

15.5x9.5mm

5

Radiant MM to Carat Conversion

Radiant cuts have a square or rectangular shape with a brilliant-cut faceting.


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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

Princess cuts are square with excellent brilliance, but they tend to appear slightly smaller than round cuts of the same carat weight.


Shop Princess Natural Diamonds | Shop Princess Lab Diamonds

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

Marquise is an elongated shape, which maximizes surface area and tends to look larger than other cuts.


Shop Marquise Natural Diamonds | Shop Marquise Lab Diamonds

Approximate MM Diamond Size

Approximate Diamond Carat Weight

8x4mm

0.5

10.5x5mm

1

11.5.7mm

1.25

11.7x6mm

1.5

13x6.5mm

2

14x7mm

2.5

15x7.5mm

3

16x8mm

3.5

16.5x8.3mm

4

17.5x6.5mm

4.5

17.7x9mm

5

Asscher MM to Carat Conversion

Asscher cuts have a unique square shape with deep step-cut facets.


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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 MM to Carat Conversion

Heart shapes, a modification of the pear shape, are measured at their widest point.


Shop Heart Natural Diamonds | Shop Heart Lab Diamonds

Approximate MM Diamond Size

Approximate Diamond Carat Weight

5.5x5mm

0.5

7x6mm

1

7.6x6.5mm

1.25

8x7mm

1.5

9x8mm

2

9.5x8.5mm

2.5

10x9mm

3

10.5x9.5mm

3.5

11x10mm 

4

11.5x7.5mm

4.5

12x10.5mm

5

MM to Carat FAQ

How many mm is a half carat diamond?

A half-carat round diamond typically measures around 5.0 mm in diameter, though this can vary slightly with the cut and proportions.

How many mm is a 1 carat diamond?

A 1-carat round diamond measures approximately 6.3 mm in diameter. Other shapes, like ovals or cushions, may appear larger or smaller based on specific dimensions. For round and square diamonds, 1 carat generally ranges from 5.5 to 6.5 mm. For elongated fancy shapes, 1 carat may vary from 7x4.5 mm to 10x5 mm.

How many mm is a 2 carat diamond?

A 2-carat round diamond generally measures about 8.0 mm in diameter. Other shapes may differ, with some elongated shapes appearing larger than round stones of the same weight. For round and square diamonds, 2 carats usually measure between 7 and 8 mm. Fancy shapes often fall between 8x6 mm and 9x7 mm, while marquise diamonds, which are particularly long, may be closer to 13x6.5 mm at 2 carats.

How many mm is a 3 carat diamond?

Round and square diamonds weighing 3 carats typically measure at least 8 mm, with most reaching around 9 to 9.25 mm. For fancy shapes, 3-carat gemstones generally range from 10x8 mm to 12x8 mm.