The Diamond Pro

Diamond Proportion

By Michael Fried
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Diamond proportion is the ratio and size of the diamond’s depth, width and table. The exact proportions like table and depth percentage play a huge role in the beauty of a diamond. Diamond proportion is an element of Diamond Cut and is critical to a stone’s brilliance and value. 

Diamond Measurements

With ideal diamond proportions, a diamond takes in and reflects light well—causing brilliance and fire. This 1.26 carat round cut diamond is a stunning example of a diamond with ideal cut proportions. 

If a diamond is cut too shallow or too deep, light is lost out the sides and doesn’t reflect as well back to the eye. In other words, if a diamond’s proportions are poor, the diamond looks dull and lifeless. 

Because a diamond with ideal cut proportions offers more brilliance, the price also tends to be higher for these diamonds. Throughout our years of experience, we find that the higher price is worth it, because cut quality impacts the diamond’s beauty more than any other feature. That’s why we suggest spending a good amount of your engagement ring budget on Cut quality, over other features like Color and Clarity

How Diamond Proportion is calculated?

Diamond proportion is calculated using the dimensions of the table size, crown height and pavilion depth in relation to the diamond’s diameter. 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 formulas for each specific component of a diamond’s proportions are listed below.

Diamond Table

Table percentage is calculated by dividing the table width (top surface area) by the diameter. For example, if the table facet is 3mm wide and the diameter is 5mm, the table percentage is 60%. 

  • If the table percentage is too low, light gets trapped inside the diamond and leaks out the sides of the diamond (instead of reflecting back through the table). 
  • If the table percentage is too high, light doesn’t reflect off of the diamond’s crown angles and facets—leaving the diamond looking dull. 

Diamond Width

Width is calculated by measuring from one end of the girdle (the diameter at its widest point) to the opposite end of the girdle.

  • Width is primarily used to determine the length to width ratio which denotes how proportionate the diamond is for its intended shape (i.e. rectangular vs. square). 
  • Length to width ratio is calculated by dividing the length of the diamond by its width. For instance, if a diamond has a length of 5mm and a width of 3mm, the length to width ratio is 1.67.

Diamond Depth

Depth percentage is calculated by dividing the depth of the diamond (its height) by its width. For example, if a diamond measures 3mm in depth and 4mm in width, the depth percentage is 75%. 

  • A diamond with a lower depth percentage usually appears larger due to its increased width, but often creates a dark appearance as the diamond doesn’t reflect light as well.
  • A diamond with too high of a depth percentage loses light out the bottom of the diamond, making it appear dull.
How Light reflects through a diamond

The table, depth and width all combine to impact how well a diamond refracts and reflects light. That’s why it’s essential to find a diamond with ideal cut proportions in each area. 

Factors affecting diamond proportion

There are several factors that impact a diamond’s proportions, including the table size and total depth. How well a diamond interacts with light is affected by the size, shape and angle of each facet. 

Here are the 10 main components that impact diamond proportion:

  • Table size: Length and width of a diamond’s table (top surface area)
  • Star length: Length of the angled facets adjacent to the table
  • Girdle thickness: Height of girdle (belt that divides the crown from the pavillion)
  • Lower girdle length: Width of the lower girdle—from the culet to where the facet connects to the girdle
  • Culet: Size of the culet (bottom tip of a diamond)
  • Total depth: Height of a diamond from culet to table
  • Pavilion depth: Height from culet to bottom of the girdle
  • Crown height: Height from the top of the girdle to the table
  • Pavilion angle: Angle of the pavillion from the bottom of girdle to the edge of a diamond pavilion
  • Crown angle: Angle of the crown from top of the girdle to edge of a diamond crown

Ideal Cut Diamond Proportions

The best diamond proportions allow light to refract and reflect back to your eyes. The ideal round diamond proportions are listed in the table below and based on expert recommendations. 

Best Diamond Proportions for Round Brilliant Diamonds<
Depth Percentage59 to 62.6%
Table Percantage54 to 57%
Girdle ThicknessThin to Slightly Thick
CuletNone to Pointed
Length to Width Ratio1.0 to 1.03

Each of these factors plays a role in the beauty and brilliance of the diamond. By staying within these ideal diamond proportions, you’ll end up with a stunning diamond that captures and reflects light. For example, this 1.32 carat round diamond is exceptionally brilliant because it’s well-proportioned. 

To find the best proportions, start by looking at GIA and AGS graded Excellent and Ideal cut diamonds. Narrow down your search to the recommendations above, as some Excellent and Ideal cut diamonds won’t meet these ranges. 

For help reviewing a specific diamond’s proportions, reach out to our experts.

Diamond Proportions Chart

Ideal diamond proportions depend on the shape, as each one interacts with light differently. As you look at diamonds, be sure you’re getting the best diamond proportions for the shape. To find these proportions, look at Excellent and Ideal cut diamonds, but remember that not all of them will meet the recommendations below. 

Best Diamond Proportions for Every Diamond Shape

ShapeDepth %Table %GirdleCuletL to W Ratio
Round Brilliant Cut59 – 62.6%54 – 57%Thin-Slightly ThickNone-Pointed1.0 – 1.03
Princess Cut68 – 74%69 – 75%Thin-Slightly ThickNone-Pointed1.0 – 1.04
Cushion Cut61 –  68%< 68%Thin-Slightly ThickNone-Pointed

1.0 – 1.08 (sq)

1.15 – 1.25 (rect)

Emerald Cut61 – 68%61 – 69%Thin-Slightly ThickNone-Pointed

1.30 – 1.45

Asscher Cut61 – 68%61 – 69%Thin-Slightly ThickNone-Pointed1.0 – 1.05
Oval Cut< 63%53 –  63%Thin-Slightly ThickNone-Pointed1.30 – 1.50
Pear Shape< 68%53 –  65%Thin-Slightly ThickNone-Pointed1.45 – 1.75
Radiant Cut< 67%61 –  69%Thin-Slightly ThickNone-Pointed

1.0 – 1.05 (sq)

1.20 – 1.50 (rect)

Heart Shape56 – 66%56 – 62%Thin-Slightly ThickNone-Pointed1.0 (or as close as possible)
Marquise Cut58 – 62%53 to 63%Thin-Slightly ThickNone-Pointed1.85 – 2.1

By staying within the recommended diamond proportions, you’ll narrow down your selection of diamonds to the very best. You’ll knock out diamonds that will look dull and lifeless in an engagement ring

Cut quality, including diamond proportions, is the most important factor in a diamond’s appearance. You’ll still want to pay attention to the other of the 4Cs: Color, Clarity and Carat.For help in choosing the right diamond, use the HD imagery on James Allen’s or Blue Nile’s websites, or reach out to our experts. We’ll take a look at the diamond you’re considering and make sure it gives you the most beauty for your budget.

About the author
Mike learned the diamond business from the ground-up at Leo Schachter Diamonds - one of the world's top diamond manufacturers. He has been recognized as a diamond industry expert by Time, PeopleMoney, The Daily Mirror, NerdWallet, The Times Herald, Yahoo Finance Australia, The Art of Charm, The Washington Diplomat, The Next Web, and more. See more
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