A Guide to Diamond Proportions
Understanding which cut qualities matter to get the ideal diamond
A diamond’s proportions are incredibly important. While it may seem like a hodgepodge of digits and percentages, if you ignore it, you may end up with a diamond that has no sparkle whatsoever. The color and clarity grades are simple enough that you may just overlook the proportions, but you do so at your own peril.
Take this one carat H VVS1 clarity round diamond from Blue Nile for example. On the face of it, it is an incredible diamond that is a whopping 33% cheaper than this one carat H VVS1 from Blue Nile. But if you start looking at the proportions, you realize that the first diamond is poor value at any price.
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.
The greatest diamond cutters in history agonized over these proportions, finding the perfect angles and facets to maximize a diamond’s brilliance. As the master diamond cutter Sir Gabriel Tolkowsky put it “The first thing I do when I hold a diamond, is I look at it and ask it, ‘what would you like to become?”
With ideal diamond proportions, a diamond takes in and reflects light well—causing brilliance and fire. This 1.26 carat round cut diamond from James Allen is a stunning example of a diamond with ideal cut proportions.
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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.
Deal Alert: Save 25% on Settings at Blue Nile!
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.
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.
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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 Percentage | 59 to 62.6% |
| Table Percentage | 54 to 57% |
| Girdle Thickness | Thin to Slightly Thick |
| Culet | None to Pointed |
| Length to Width Ratio | 1.0 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 from Blue Nile is exceptionally brilliant because it’s well-proportioned.
To find the best proportions, start by looking at GIA graded Excellent 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
| Shape | Depth % | Table % | Girdle | Culet | L to W Ratio |
|---|---|---|---|---|---|
| Round Brilliant Cut | 59 – 62.6% | 54 – 57% | Thin-Slightly Thick | None-Pointed | 1.0 – 1.03 |
| Princess Cut | 68 – 74% | 69 – 75% | Thin-Slightly Thick | None-Pointed | 1.0 – 1.04 |
| Cushion Cut | 61 – 68% | < 68% | Thin-Slightly Thick | None-Pointed | 1.0 – 1.08 (sq) 1.15 – 1.25 (rect) |
| Emerald Cut | 61 – 68% | 61 – 69% | Thin-Slightly Thick | None-Pointed | 1.30 – 1.45 |
| Asscher Cut | 61 – 68% | 61 – 69% | Thin-Slightly Thick | None-Pointed | 1.0 – 1.05 |
| Oval Cut | < 63% | 53 – 63% | Thin-Slightly Thick | None-Pointed | 1.30 – 1.50 |
| Pear Shape | < 68% | 53 – 65% | Thin-Slightly Thick | None-Pointed | 1.45 – 1.75 |
| Radiant Cut | < 67% | 61 – 69% | Thin-Slightly Thick | None-Pointed | 1.0 – 1.05 (sq) 1.20 – 1.50 (rect) |
| Heart Shape | 56 – 66% | 56 – 62% | Thin-Slightly Thick | None-Pointed | 1.0 (or as close as possible) |
| Marquise Cut | 58 – 62% | 53 to 63% | Thin-Slightly Thick | None-Pointed | 1.85 – 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.
What we love about them:
- No questions asked returns within 30 days of shipment. James Allen will send you a paid shipping label to return the ring.
- Lifetime Warranty
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- Free prong tightening, repolishing, rhodium plating and cleaning every 6 months
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- One free resizing within 60 days of purchase
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What we love about them:
- No questions asked returns within 30 days of shipment. Blue Nile will send you a paid shipping label to return the ring.
- Lifetime Warranty
- Free Shipping
- Free prong tightening, repolishing, rhodium plating and cleaning every 6 months
- Provide insurance appraisal
- One free resizing within the first year of purchase
- High quality images of about half of their diamonds
- 24/7 Customer Service
- 100% credit towards future upgrades (must be at least double in value)
- Best in class fulfillment
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