Proportions and ratio of a Diamond


The proportion of the diamond is the ratio of the size of the depth, width and table of the diamond. The exact proportions between the board and the percentage of depth play a huge role in the beauty of a diamond. The proportion of the diamond is an element of the cut of the diamond and is crucial to the brilliance and value of a stone.

With perfect proportions, a diamond welcomes and reflects light well, causing brilliance and fire.

If a diamond is cut too flat or too deep, the light is lost from the sides and is not reflected. In other words, if the proportions of a diamond are poor, the diamond appears dull and lifeless. Since a diamond with ideal cut proportions offers more brilliance, the price also tends to be higher for these diamonds and, over our years of experience, we have found that it is worth the extra money for the cut quality. since this affects the beauty of the diamond more than any other characteristic.


The proportion of the diamond is calculated using the dimensions of the table, the height of the crown and the depth of the pavilion in relation to the diameter of the diamond. Each calculation describes the angles and ratios of the diamond’s precise cut. These aspects are universally measured in millimeters (mm) and / or percentages.


The plank percentage is calculated by dividing the plank width (top surface area) by the diamond diameter. For example, if the facet of the board is 3mm wide and the diameter is 5mm, the percentage of the board is 60%.

If the percentage of the table is too low, the light is trapped inside the diamond and escapes from the sides of the diamond (instead of reflecting through the table). If the percentage of the table is too high, the light does not reflect on the corners and facets of the crown thus leaving the diamond dull.


The width is calculated by measuring the two ends of the belt.

Width is mainly used to determine the length-to-width ratio which denotes how proportionate the diamond is to its intended shape (i.e. rectangular versus square). The length to width ratio is calculated by dividing the length of the diamond by its width. For example, if a diamond is 5mm long and 3mm wide, the length / width ratio is 1.67.


The depth percentage is calculated by dividing the diamond’s depth (its height) by its width. For example, if a diamond is 3mm deep and 4mm wide, the depth percentage is 75%.

A diamond with a lower depth percentage usually appears larger due to its greater width, but often creates a darker appearance as the diamond does not reflect light. A diamond with too high a percentage of depth loses light on the bottom of the diamond, making it appear dull.

The best proportions of the diamond allow for better refraction of light. The ideal proportions of the round diamond are listed below and based on expert recommendations.


There are several factors that affect the proportions of a diamond, including the size of the table and the total depth. The way in which a diamond interacts with light is influenced by the size, shape and angle of each facet.

Here are the 10 main components that affect the proportion of the diamond:

Table size: length and width of a diamond table (top surface)

Star length: length of the angled facets adjacent to the table

Belt thickness: Belt height (the belt divides the crown from the pavilion)

Bottom girdle length: The width of the bottom girdle, from the butt to where the facet connects to the girdle

Butt: diamond butt size (bottom tip of a diamond)

Total depth: height of a diamond from butt to table

Pavilion Depth: Height from the butt to the bottom of the girdle

Crown Height: Height from the top of the girdle to the table

Pavilion angle: angle of the pavilion from the bottom of the girdle to the edge of the diamond pavilion

Crown angle: angle of the crown from the top of the girdle to the edge of the diamond crown


Le migliori proporzioni del diamante consentono una migliore rifrazione della luce. Le proporzioni ideali del diamante rotondo sono elencate a seguito e basate sulle raccomandazioni degli esperti.

Percentage of Depth: 59% to 62.6%

Percentage of the table: from 54% to 57%

Belt thickness: From thin to slightly thick

Butt: nothing or pointed

Length-to-width ratio: 1.00 to 1.03

Each of these factors plays an important role in the beauty and brilliance of the diamond and by remaining within these ideal proportions we will thus have an extraordinary diamond that captures and reflects light in its maximum splendor.

To find the best proportions in Excellent and Ideal cut diamonds, it is necessary to check the classifications of GIA and AGS. These are the two most serious and reliable diamond grading entities in the world. Both provide objective and accurate assessments of a diamond’s quality, without exaggeration or embellishment.

Below is an explanatory table that will help you in the best choice to identify the right proportions of each diamond in its different shapes.

The best diamond proportions for each shape its shape

Diamond cut

Percentage of Depth

Percentage of the board

Thickness of the Belt


Length / width ratio

Round Brilliant

(59 – 62.6) %

(54 – 57) %

Thin to slightly thick

Nothing or Pointed

1.00 – 1.02


(71 – 75) %

(68 – 72) %

Thin to slightly thick

Nothing or Pointed

1.00 – 1.04


(67 – 70) %

(59 – 63) %

Thin to slightly thick

Nothing or Pointed

1.00 – 1.08 (quadro)
1.15 – 1.25 (rettangolo)


(66 – 69) %

(61 – 65) %

Thin to slightly thick

Nothing or Pointed

1.35 – 1.50


(66 – 69) %

(61 – 65) %

Thin to slightly thick

Nothing or Pointed

1.0 – 1.04


(60 – 63) %

(56 – 60) %

Thin to slightly thick

Nothing or Pointed

1.37 – 1.55


(60 – 63) %

(56 – 60) %

Thin to slightly thick

Nothing or Pointed

1.55 – 1.70


(67 – 70) %

(64 – 68) %

Thin to slightly thick

Nothing or Pointed

1.00 – 1.05 (square)
1.25 – 1.40 (rectangle)


(56 – 59) %

(56 – 60) %

Thin to slightly thick

Nothing or Pointed

1.00 – 1.20
(best as close as possible to 1.00)


(60 – 63) %

(56 – 60) %

Thin to slightly thick

Nothing or Pointed

1.90 – 2.10

Diamonds are famous for their brilliance and each diamond shines under certain lighting conditions, some diamond cuts or shapes are more likely to have a pronounced and very noticeable sparkle than others. While round diamonds will have more brilliance, other factors can make a diamond shine producing the most impressive sparkle, brilliance and fire.


The sparkle of a diamond depends on its ability to absorb light and therefore reflect it outwards. The more light the diamond reflects from its facets, the brighter it will appear.
To do this, diamonds are cut into shapes consisting of numerous facets. Each facet acts in a similar way to a mirror, reflecting the light from one side of the diamond to the other, making it bounce off the diamond.
Now, it is important to understand what “sparkle” means in this sense, as well-cut diamonds can reflect different types of light. This ability to reflect light is generally referred to as “brilliance” and “fire”.
Brilliance refers to a diamond’s ability to reflect white light. The more white light a diamond can reflect from its table, the brighter it is. This is the factor that makes a diamond shine and it looks particularly impressive and beautiful.
Fire refers to a diamond’s ability to disperse colored light. A diamond with a large fire could emit a colorful reflection, giving it a unique character and beauty. Fire can take the form of any of the spectral colors, from red to orange, yellow, green, blue and purple.
More often than not, diamond cuts with more facets are brighter than diamond cuts that have fewer facets. Some diamond cuts are specially designed to have the ideal number of facets for optimal reflection and brilliance of light. An exquisitely cut round diamond will light up like a strobe light, however if you decide to go for an emerald cut it will still be beautiful but will not have the same brilliance as a round one.


The brightest or most sparkling cut diamond of all is the round brilliant cut. Round brilliant diamonds are cut to have 58 facets (including butt) allowing light to enter the diamond and reflect off each facet to create a beautiful sparkle.

Interestingly, the round brilliant diamond cut was actually designed specifically to accomplish this. In 1919, Marcel Tolkowsky, engineer and diamond cutter, published a new mathematical formula for obtaining optimal diamond brilliance as his doctoral thesis. The formula led to the creation of the round brilliant cut, the most popular in the world.


These cuts are all famous for their brilliance. They are great alternatives that offer a similar level of sparkle and fire at a lower cost per carat than a round brilliant cut diamond.


The princess cut has a great ability to cut and reflect light. In a high cut quality, a princess cut diamond can have an absolutely stunning brilliance.


The radiant cut has been specially developed to combine the best aspects of the round brilliant cut and the emerald cut, with 70 facets giving it a great ability to reflect light and create a beautiful sparkle.


The cushion cut is very good at reflecting light, giving a high level of brilliance and fire to the diamond. While it doesn’t match the round brilliant cut perfectly, it also offers strong brilliance at a significantly lower price per carat.


Although the round brilliant cut has the strongest sparkle, it is not the only diamond cut capable of capturing and reflecting light. Several other diamond cuts share the characteristics of the round brilliant cut, including a facet pattern designed to reflect as much light as possible. These include the oval, marquis and pear cuts.


Oval diamonds as per their name are cut into an oval shape, with a faceted pattern similar to the round brilliant cut. As is the marquis also with an elongated shape, the oval cut is one of the largest cut diamonds compared to the same carat weight. There are 56 facets on an oval diamond, allowing this cut to show a stunning sparkle and fire when cut well.


Like the marquis and oval cuts, the pear (or, more commonly called teardrop) cut has a similar facet to the round brilliant cut. In fact, just like the round brilliant cut, a typical pear-shaped diamond has 58 facets, which allow it to absorb and reflect an excellent amount of light. Unfortunately many teardrop diamonds are poorly cut, resulting in a faint sparkle. If you are looking for a pear-shaped diamond and want to avoid buying one of poor brilliance, do not hesitate to contact us for free assistance.


Heart cut diamonds are heart shaped, cut similar to brilliant cut diamonds, with 56/58 facets. This means that when cut right, they can have a truly astounding level of brilliance and fire. Like other fancy shaped diamonds, finding a well cut heart shaped diamond can be a challenging process. Again, if you are interested in a heart shaped diamond and are unsure what to look for, feel free to contact us for expert advice.


The Marquis cut was developed in the 18th century when King Louis XV of France requested a diamond cut in the shape of the lips of his mistress, the Marquise Madame de Pompadour, Jeanne Antoinette Poisson.

With 56 facets, the Marquis cut has a facet pattern similar to the round brilliant cut. This gives it an excellent brilliance, which means that a well-cut Marquis diamond will offer great brilliance. In addition to its impressive brilliance, the marquis cut tends to appear slightly larger than different cut diamonds of the same carat weight. This makes it an option worth considering if you want a diamond that combines a beautiful sparkle with an impressive presence.


While beautiful in their own right, the Asscher and Emerald cuts are not well known for their brilliance. Due to their design and facet pattern, diamonds in these shapes are generally not as bright and sparkling as those with a brilliant cut based shape.

However, these shapes are valued for their clarity and luster. Well cut and with a high degree of purity, a high quality Asscher or Emerald cut diamond can have a unique and elegant appearance.

In general, diamonds with a large number of facets such as those listed above will shine more than diamonds with fewer facets.

However, the shape of a diamond is only one of the many factors that can determine the strength with which it shines. Other factors, such as the quality of the cut, the ratio of length to width, and the ratio of depth to table, affect a diamond’s brilliance, focus, and overall ability to shine.

Cut quality has the greatest impact on a diamond’s brilliance and fire. Simply put, a well cut diamond will reflect much more light than a poorly cut one, resulting in a much stronger and more visible sparkle. For a round brilliant cut diamond, limit your selection to “Excellent” (if the diamond is rated by GIA) or “Ideal” (if it is rated by AGS). These cut grades ensure that the diamond has the right proportions for optimum brilliance and fire. If you are considering a fancy shape diamond, we recommend that you use the length / width ratio and depth / table ratio guidelines in our diamond shape guide to avoid buying a diamond that does not reflect light well.

The advice is to stick to diamonds with GIA or AGS certification

As mentioned above, it is important to stick to diamonds with an Excellent (GIA) or Ideal (AGS) cut grade, these are the two most serious and reliable diamond grading entities in the world. Both provide objective and accurate assessments of a diamond’s quality, without exaggeration or embellishment.

Other valuation entities are not as reliable and often provide artificially high valuations which make a diamond look better than it actually is. Although these generally affect the color and clarity more than the cut, it is still best to stick to diamonds that come with a GIA or AGS certificate to make sure you are really getting what you expect.

If you are looking for the most sparkling diamond we recommend sticking to the round brilliant cut.
Other diamond shapes also reflect a lot of light, such as marquis, oval, pear, heart cuts and can shine beautifully. Princess, radiant and cushion cuts are also great when cut to a high standard and are all great choices.

That said, there is more to a diamond’s sparkle than its shape. Look for a diamond that has an Ideal (GIA) or Excellent (AGS) cut grade, as it is more likely to reflect light and produce a beautiful brilliance.