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

Many customers ask us how to measure a gemstone's size versus a diamond's as moissanite and other gemstone rings gain popularity as alternatives to traditional diamond engagement rings. We've put together some useful advice to help address some of the most common queries from our customers regarding gemstone mm to carat conversion!

To ensure you get the best value for your money, all diamonds at LBJ are listed with comprehensive specifications, including weight, measurement, cut quality, and imaging.

How Is a Diamond's Size Determined?

Most people know that diamonds are measured in units called "carats," but many may not be aware that carats measure weight rather than dimension. Carat weight does not equate to size, even though it affects how your diamond appears and feels.

How Is a Diamond's Size Determined?

Two diamonds with the same carat weight may appear differently depending on how they carry their weight. For instance, compared to traditional round diamonds of equivalent carat weight, elongated fancy-shaped diamonds with their longer dimensions and shallower depths may appear to occupy more space in a ring or on a finger.

For instance, elongated fancy-shaped diamonds may appear to occupy more space in a ring or on a finger than traditional round diamonds of the same carat weight because of their longer dimensions and shallower depths.

Poor diamond cutting can also impact A diamond's appearance, which can retain excess weight "hidden" beneath the diamond's girdle, part of the stone normally hidden by the ring. For that reason, to the unaided eye, a 1.25-carat round diamond with poor cutting may seem the same size as one with excellent cutting.

Nevertheless, carat weight can help you understand how big or small your diamond will seem. One Carat is divided into 100 "points," 200 milligrammes, or 0.2 grammes. Because of this, gemologists can provide incredibly accurate measurements down to the hundredth decimal place.

Since carat weights are written out in the same manner as dollars and cents, it's easy to think of them that way! 2.25 ct, or two carats and twenty-five points, equals $2.25 and twenty-five cents. With cut, colour, and clarity, Carat is one of the traditional "4 C's" of diamond grading that the GIA developed as a standard method of determining a diamond's quality.

Weight of Gemstones Compared to Size

At LBJ, we classify gemstones based on their millimetre dimensions, although some jewellers weigh gems in carats other than diamonds. Different gemstones have varying densities, making some gems appear different from diamonds with similar carat weights. As we just discussed, Carat is a unit of measurement for weight rather than size. Since Carat was developed especially as a unit of measurement for diamonds, it makes sense to treat other gemstones differently.

Denser than diamonds are gemstones such as garnet, zircon, sapphires, and rubies. Notable gems considerably less dense than diamonds include moissanite, topaz, jade, peridot, emerald, tourmaline, morganite, amethyst, citrine, opal, and aquamarine. Because rubies are inherently denser and heavier than diamonds, a 1.5-carat round ruby will appear smaller than a 1.5-carat round diamond.

Comparing the mm dimensions of gemstones to the carat weights of diamonds is more of an approximation than an exact science due to their differing densities. The approximate diamond carat weight comparisons for the most popular gemstone sizes that we offer are displayed in the charts below.

Weight of Gemstones Compared to Size

Round and perfectly square gems will have a single listed measurement, such as "8mm," because they will have the same diameter from every angle. The Length and Width of gemstones in other shapes will be expressed in millimetres. Examining the length-to-width ratio is useful for those attempting to achieve a specific look.

For instance, depending on their length-to-width ratios, radiant cut gems may appear more square-shaped or more rectangular. A radiant cut gem with a 2:1 ratio will appear significantly more elongated than one with a 1:1 ratio, which will appear square. Ovals and pears are two other fancy shapes whose ratios can make them appear more slender or round.

Chart To Convert MM To Carat For Round Diamonds And Gemstones

Undoubtedly, round-cut diamonds are the most preferred option for exquisite jewellery. The round-cut diamond has a long and distinguished history. Over the years, experts and cutters of diamonds have studied their light performance for decades, perfecting exact formulas to increase the diamond's brilliance, scintillation, and fire.

This shape is the most traditional and well-liked option because of its breathtaking beauty and brilliance.
The circumference of a round brilliant diamond is roughly the same as that of any other circle.
When viewing the stone from above, an imaginary line connecting two points across from one another on the girdle gives its diameter.

Brilliant diamonds have a constant diameter throughout, just like any other circle. Because of its exquisite beauty and remarkable brilliance, this shape is by far the most traditional and well-liked option.

Jewellers, wholesalers, and individual clients frequently ask us how to convert a diamond's millimetres to carat weight and vice versa. We have created a straightforward millimetre-to-carat weight conversion chart for the most common diamond shapes. With the help of this chart, you can select the centre stone of a diamond engagement ring or pendant, as well as the ideal side stones to go with it.

Additionally, we can arrange layouts to help match side stones to the centre stone and each other for customers' customised jewellery pieces. Please be aware that these conversions are approximations derived from in-depth investigation on our part.

The round MM size weighs 4 mm or 0.25 ct.

4.5 mm, 0.36 ct.
5 mm,0.5 ct.
5.5 mm,0.66 ct.
5.75 mm 0.75 ct.
6 mm,0.84 ct.
6.5 mm,1 ct.
7mm,1.3 ct.
7.5 mm,1.67 ct.
8 mm, 2 ct.
8.5 mm,2.11 ct.
2.75 ct,9 mm.
3.35 ct,9.5 mm.
3.87 ct,10 mm.
10.5 mm,4.41 ct.
11 mm,4.91 ct.

Chart To Convert MM To Carat For Oval Diamonds And Gemstones

Oval diamonds and gemstones are brilliant and attractive options in demand. Their oblong shape becomes longer and longer, giving them a similar sparkle to round diamonds.

MM Size OvalCarat weight of oval, 6 x 5 mm, 0.61 ct.

7.7x5.7 mm,1 ct.
8x6mm,1.25 ct.
9x6mm,1.41 ct.
9x7mm,2 pieces.
10.x7 mm,2.18 ct.
2.5 ct,10x8 mm.
3 pieces,12 x 8 mm.
3.85 count,11x9 mm.
5.05 pieces,12x10 mm.

Chart To Convert MM To Carat: Emerald Diamonds And Gemstones

The most common shape of a step-cut diamond is emerald. Their sleek design, which highlights the colour and clarity of a gemstone, is created by their rectangular facets and clean lines.

Emerald Millimetre SizeEmerald Carat Weight: 1 carat, 7x5 mm.

1.5 ct,7.5x5.5 mm.
1.75 count, 8x6 mm.
9x7mm,2.5 ct.
3 pieces, 10 x 7.5 mm.
11.x9mm,5.21 ct.
12x10mm,6 pieces.

Princess Diamonds and Gemstones: MM To Carat Conversion Chart

The princess cut diamond has a modern appearance and classic performance because it skilfully combines traditional diamond sparkle with a modern silhouette. Though it was first added to the diamond-shaped menu in the 1960s, it is still relatively new.

The princess cut, second only to the round cut in popularity, offers brides brilliance, scintillation, and something unique.
Modified brilliant cuts, also known as square or rectangular faceting patterns, are the group of faceting patterns that make up princess-cut diamonds.

Based on the dimensions used to communicate them, princess carat weights can be somewhat misleading in terms of visual size compared to rounds. The depth percentage of a round, which is based on its Depth, diameter, and girdle thickness, is a key factor in determining its footprint or "spread".

We determine a princess cut diamond's dimensions and depth percentage by taking the smallest width-to-total depth ratio; this yields a comparatively larger depth and smaller dimension. It's crucial to understand that the perception that princess cuts appear smaller than rounds carat for Carat is inflated by this calculation method.

For many years, princess diamonds have been the most popular fancy shape. These stones usually cost less and have the same brilliance in a square shape as round diamonds of the same carat weight or size.

Princess MM Dimensions Princess Carat:

5.5 mm,1 ct.
1.25 ct,6 mm.
1.5 ct,6.5 mm.
7 mm,2 ct.
2.5 ct,7.5 mm.
8mm,3 ct.
9 mm,4.12 ct.

Chart To Convert MM To Carat For Pear-Cut Diamonds And Gemstones

Pear diamonds, also called "teardrop" diamonds, combine the elongated points of marquise diamonds with the sparkle and light play of round diamonds.

Pear MM Dimensions Carat Weight of Pears

7x5mm,0.75 ct.
7.5x5.5mm,0.85 ct.
7.7x5.7 mm,1 ct.
8x6mm,1.25 ct.
1.5 ct,8.5x6.6 mm.
10x6mm; 1.71 ct.
9x7mm,2 pieces.
2.5 ct; 10x8 mm.
11x8mm,2.7 ct.
3 pieces,12 x 8 mm.
13x8mm,3.4 ct.

Chart To Convert MM To Carat: Cushion Cut Diamonds and Gemstones

Cushion-cut diamonds and gemstones are a classic option that looks especially good in the halo and vintage-style ring settings.

MM Size CushionCushion Carat:

5.5 mm,1 ct.
1.25 ct,6 mm.
1.5 ct,6.5 mm.
7 mm,2 ct.
2.5 ct,7.5 mm.
8mm,3 ct.
9 mm,4.12 ct.
10. mm,5.62 ct.
12mm,9.52 ct.
11mm,7.44 ct.

MM To Carat Conversion Table: Gemstones And Marquise Diamonds

The navette, or "little boat," is another name for this shape. The carat weight-to-mm conversion of marquise diamonds and gemstones will be the most unusual on this list because they typically have a noticeable length-to-width ratio.

Size of Marquise MMMarquise Carat Weight: 0.3 carats, 7x3 mm.

8x4mm,0.5 ct.
9x4.5mm,0.75 ct.
10x5mm,1 piece.
11x6mm,1.33 ct.
1.5 ct,12 x 6 mm.
13x6.5mm,2 ct.
2.5 ct,14x7mm.
3 pieces,15x7mm.

Chart To Convert MM To Carat For Asscher Diamonds And Gemstones

Asscher gems are square-shaped stones distinguished by their smooth, X-shaped step-cut facets.

Asscher MM DimensionsOne carat of asscher weights 5.5 mm.

1.25 ct,6 mm.
1.5 ct,6.5 mm.
7 mm,2 ct.
2.5 ct x 7.5 mm.
8mm,3 ct.
3.5 ct,8.5 mm.
9 mm < 4.12 ct.
10. mm, 5.62 ct.

Chart For Converting MM To Carat: Shining Gemstones And Diamonds

Although stunning radiant diamonds can be square or rectangular, we have concentrated on the dimensions for elongated radiant gems in this chart. Cushion or princess gems can be converted similarly to square radiant gems.

Radiant Millimetre DimensionsRadiant Carat Weight: 0.5 carats, 6x4 mm.

One piece,7x5 mm.
7.3 x 5.5 mm,1.25 ct.
1.5 ct,7.5x5.8 mm.
8x6mm,2 pieces.
9x7mm,2.5 ct.
10x7mm,3.31 ct.
10x8mm,3.5 ct.
5.05 pieces, 11x9 mm.

Chart To Convert MM To Carat: Heart-Shaped Diamonds And Gemstones

Heart-shaped gems are a unique and romantic take on the pear shape. Heart-shaped gems are measured by measuring their widest point, not the length-to-width ratio, even though they are not precisely round or square.

Heart Millimetre DimensionsWeight of the Heart Carat

5 mm,0.5 ct.
5.51 ct,0.61 ct.
6 mm,0.75 ct.
6.5 mm,1 ct.
7 mm,1.25 ct.
1.5 ct,7.5 mm.
8 mm per,2 ct.
2.5 ct,8.5 mm

Frequently Asked Questions Concerning Conversions From MM To Carat

One Carat Is How Many MM?

Like round and square diamonds, one Carat is roughly equivalent to 5.5–6.5 mm for diamond shapes that are equally spaced from top to bottom and side to side. Fancy diamonds can range from 7 by 5 mm to 10 mm in one Carat (one full Carat).

Two Carats Is How Many MM?

Two carats normally have between 7 and 8 mm dimensions for round and square diamonds. Two carats are equivalent to between 8x6mm and 9x7mm in fancy-shaped diamonds. Marquise diamonds are especially long and thin at two carats and will measure closer to 13x6.5mm.

Three carats is how many MM?

Three-carat round and square diamond shapes will measure at least 8 mm, but more likely 9 or 9.25 mm. Depending on their shape, 3-carat fancy diamonds can have a wide range of millimetre measurements; however, most fall between 10x8mm and 12x8mm.

How is a gemstone's carat weight determined?

Gemstone Purchasing Guide | ICONIC - Carat Weight & Gemstone Size
Weighing the gemstone and recording its weight in grammes is the first step towards determining its accurate carat weight. Next, calculate the carat weight by dividing the grammes by 0.2. Is Carat a unit of measurement for size or weight? A carat (ct) is the weight unit used to measure diamonds and other gems.

What is the diamond size calculation formula?

The following are the formulas: The formula for calculating diamond carats is Length x Width x Depth x Coefficient = 5 x 3.5 x 2.5 x 0.0090 = 0.394 carats (or about 39 satang) (points).

How is the size of a diamond determined?

Carat weight, which equals 0.2 grammes, is a unit of measurement for diamond size. On the other hand, dimensions, like Length, Width, and Depth in millimetres, define size more precisely. The diameter, or top of, a diamond can indicate its apparent size, particularly when two similar diamonds are being compared.

Does a diamond's carat weight alone determine how big it looks?

No, a diamond's size can also be influenced by other elements like its cut, shape, and setting. A poorly cut diamond may appear more petite, while a well-cut diamond may appear larger than its carat weight indicates.

Does a diamond's carat weight alone determine how big it looks?

Can a diamond look larger in its setting?

The centre stone can appear more prominent by having smaller diamonds set in halo settings around it. The bezel and prong settings can also influence the perception of the diamond's size. And last, if you set your diamond in a thin or tiny band, it will look larger.

Are there moral issues to take into account when selecting a diamond size?

Because large diamonds are more valuable and likely to be examined more closely for ethical sourcing, don't forget to ask about the diamond's provenance and conflict-free status. Some choose lab-grown diamonds as the more morally superior option for larger stones.

Final remarks

Our knowledgeable jewellery specialists are always pleased to answer enquiries and offer imagery if you need further assistance picturing the sizes of diamonds or gemstones. You can schedule a virtual appointment, live chat with one of our specialists on our website, or visit them in person at one of our showrooms. If you are concerned about the sizing of diamonds and other gemstones.