How to Find MM to Carat Weight Conversions for Diamonds and Gemstones

As moissanite and other gemstone rings become popular alternatives to classic diamond engagement rings, we get many customers asking how to measure a gemstone’s size vs. a diamond’s. To help answer some of our customers’ most frequently asked questions on gemstone mm to carat conversion, we’ve pulled together some helpful tips!

How do you measure the size of a diamond?

Most people are aware that diamonds are measured using a unit called “carats,” but what many may not understand is that carats are a measurement of weight and not of dimension. While carat weight does impact the overall look and size of your diamond, carat does not equal size. In fact, two diamonds of the same carat weight can have a different appearance based on how they carry their weight. For example, elongated fancy shaped diamonds that have shallower depths and longer dimensions can appear to take up more real estate in a ring or on a finger than classic round diamonds of comparable carat weight. Poor diamond cutting can also affect the appearance of a diamond and keep extra weight “hidden” below the diamond’s girdle – the base of the diamond that is typically covered up by the ring. So, a poorly cut 1.25 ct. round diamond may appear to be the same size as a well cut 1 ct. round diamond to the naked eye.

That being said, carat weight can still give you a good idea for how large or small your diamond will appear. One carat equals 200 milligrams, or 0.2 grams, and is subdivided into 100 “points.” This allows gemologists to give extremely precise measurements to the hundredth decimal place. An easy way to think of carat weight is by comparing them to dollars and cents – they’re even written out the same way! $2.25 means two dollars and twenty-five cents, and 2.25 ct. means two carats and 25 points. Carat is one of the classic “4 C’s” of diamond grading invented by the GIA as a standard way of measuring a diamond’s quality, along with cut, color, and clarity.

Gemstone Weight vs. Gemstone Size

While there are jewelers who will measure gems other than diamonds in carat weight, at Brilliant Earth we choose to categorize gemstones by their millimeter dimensions instead. This is because different gemstones have different densities, meaning that certain gems will have a different appearance to diamonds of comparable carat weight.  As we mentioned above, carat is a measurement of weight and not size. Carat was also invented as a unit of measurement specifically for diamonds, so it makes sense to use a different approach for other gemstones.

Gemstones like rubies, sapphires, zircon, and garnet are all denser than diamonds. Other popular gems like moissanite, topaz, jade, peridot, emerald, tourmaline, morganite, amethyst, citrine, opal, and aquamarine are all significantly less dense than diamonds. So, a 1.5 ct. round ruby will look smaller than a 1.5 ct. round diamond because rubies are naturally denser and heavier than a diamond.

Because of their different densities, comparing gemstone mm dimensions to diamond carat weights is more of an approximation than an exact science. The charts below show the approximate diamond carat weights comparison for the most popular gemstone sizes we offer.

Round and perfectly square gems will have the same diameter from all angles and will therefore have one measurement listed, for example, “8mm.” Gemstones in other shapes will have both their length and width listed in millimeters. Looking at the length to width ratio is a helpful tool for those trying to achieve a certain look. For example, radiant cut gems can look more square or more rectangular based on their length to width ratios. A radiant cut gem with a 1:1 ratio will look square, but one with a 2:1 ratio will look much more elongated. Other fancy shapes like ovals and pears can look more rounded or slender based on their ratios.

MM to Carat Conversion Chart: Round Diamonds and Gemstones

Like all circles, round brilliant diamonds will have the same diameter all the way around. This shape is by far the most classic and popular choice due to its spectacular brilliance and beauty.

MM to Carat Conversion Chart: Oval Diamonds and Gemstones

Brilliant and flattering, oval diamonds and gemstones are becoming an increasingly sought-after choice. They have a similar sparkle to round diamonds, but with a lengthening oblong shape.

MM to Carat Conversion Chart: Emerald Diamonds and Gemstones

Emerald diamonds are the most popular step cut diamond shape. Their clean lines and rectangular facets create a chic look that shows off a gemstone’s clarity and color.

MM to Carat Conversion Chart: Princess Diamonds and Gemstones

Princess diamonds have been by far the most favored fancy shape for decades. These stones offer the brilliance of a round diamond in a square shape, and usually for a lower price point for the same carat weight or size.

MM to Carat Conversion Chart: Pear Diamonds and Gemstones

Also known as “teardrop” diamonds, pear diamonds marry the sparkle and light-play of round diamonds with the elongated points of marquise diamonds.

MM to Carat Conversion Chart: Cushion Diamonds and Gemstones

Cushion diamonds and gemstones are a timeless choice and are particularly well-suited to halo and antique-inspired ring settings.

MM to Carat Conversion Chart: Marquise Diamonds and Gemstones

This shape is also referred to as the navette, or “little boat.” Because marquise diamonds and gemstones tend to have a pronounced length to width ratio, their carat weight to mm conversion will be the most unique on this list.

MM to Carat Conversion Chart: Asscher Diamonds and Gemstones

Asscher gems are square shapes known for their sleek step cut facets that form an X from corner to corner.

MM to Carat Conversion Chart: Radiant Diamonds and Gemstones

Dazzling radiant diamonds can be square or rectangular, but for the purposes of this chart we have focused on the dimensions for elongated radiant gems. Square radiant gems will have similar conversions to cushion or princess gems.

MM to Carat Conversion Chart: Heart Diamonds and Gemstones

Distinctive and romantic, heart shaped gems are a modification of the pear shape. While they are not perfectly round or square, heart shaped gems are measured using their widest point rather than with a length to width ratio.

Frequently Asked Questions about MM to Carat Conversions

How many MM is 1 carat?

For diamond shapes that are equidistant from top to bottom and side to side such as round and square diamonds, one carat equals approximately 5.5-6.5mm. For more elongated fancy diamond shapes, 1 ct. can be anywhere from 7x5mm to 10x5mm.

How many MM is 2 carats?

For round diamonds and square diamonds, 2 carats will typically be between 7 and 8mm in dimension. For fancy shaped diamonds, 2 carats are between 8x6mm and 9x7mm. Marquise diamonds, which are particularly long and thin, will be closer to 13×6.5mm at 2 carats.

How many MM is 3 carats?

Round and square diamond shapes that weigh in at 3 carats will be at least 8mm, but most likely closer to 9 or 9.25mm. 3 carat fancy diamonds come in a variety of millimeter dimensions based on their shape, but most are between 10x8mm and 12x8mm.

Final Thoughts

For more help visualizing the sizes of diamonds or gemstones, our expert jewelry specialists are always happy to answer questions and provide imagery. You can visit one of our specialists in person at one of our showrooms, by booking a virtual appointment, or by chatting with them live on our site. Have more questions about diamond and gemstone sizing? Let us know in the comments below!