Diamond Cut Grade Proportion Calculator

Diamond Cut Grade Proportion Calculator – GIA Style Geometry Insight

Cut is the most visually important of the diamond “4Cs” because it controls how efficiently a stone returns light to your eye. The Diamond Cut Grade Proportion Calculator focuses on the geometric part of cut: table percentage, depth percentage, crown angle, pavilion angle, girdle thickness and culet size. By feeding in the measurements from a lab report, you can see whether a round brilliant diamond sits in the Excellent, Very Good, Good, Fair or Poor range in a GIA-style proportion model.

GIA’s actual cut grade is proprietary and based on millions of modeled diamonds, but the public knows the approximate windows that tend to produce top performance. This calculator takes those windows, applies them to each proportion and then uses the weakest area to set the overall cut grade. The goal is not to replace a grading report, but to make the underlying geometry understandable.

Key Proportion Formulas For Round Brilliant Diamonds

Several proportions are expressed as percentages so that diamonds of different sizes can be compared fairly. The base formulas relate physical measurements in millimeters to percentages.

The table diameter is the width of the large flat facet on top of the diamond. The average girdle diameter is the average of minimum and maximum diameters measured across the girdle.

Total depth is the distance from the table to the culet. Depth percentage describes how tall the diamond is relative to its width. Stones that are too shallow or too deep can leak light and lose brilliance.

These angles are expressed in degrees and determine how light bends as it travels through the diamond. The crown and pavilion have to work together to bounce light back to the viewer instead of letting it escape.

GIA-Style Excellent Ranges For Proportions

In this model, Excellent proportion ranges are based on well-known windows for round brilliant diamonds. A stone can only receive an Excellent proportion grade if all main parameters stay inside these bands.

Girdle and culet are treated qualitatively. For an Excellent result, girdle thickness should normally run from thin to slightly thick and culet should be none, very small or small. Extremely thin girdles can chip easily and very thick girdles trap weight without adding spread, while large culets can appear as dark spots in the table.

Stepwise Proportion Grading From Excellent To Poor

Each parameter is graded separately by comparing it to nested ranges. The logic is similar for depth, table, crown angle and pavilion angle.

For example, depth percentage ranges in this calculator are split as follows.

Table percentage, crown angle and pavilion angle use similar nested bands with Excellent, Very Good and Good windows centered on the most balanced combinations for light return.

How The Overall Cut Grade Is Determined

Once all subgrades are assigned, the calculator uses the weakest one to set the overall cut grade. The reasoning is simple: a single severe proportion problem can restrict how well the diamond can perform, even if other measurements are ideal.

For instance, a diamond might have Excellent depth, table and crown angle, but a pavilion angle that falls into the Good range. In that case, the overall cut grade cannot be better than Good in this model because pavilion angle has a strong influence on light leakage.

Girdle And Culet Descriptions And Their Impact

Girdle and culet are entered as descriptive categories and then mapped to grading levels.

• Girdle described as thin, medium or slightly thick is treated as Excellent or Very Good depending on the combination.
• Very thin girdles increase the risk of chipping and usually limit the grade to at most Good.
• Thick, very thick or extremely thick girdles may trap unnecessary weight and limit the grade to Good or Fair.
• Culet of none, very small or small is compatible with Excellent proportions.
• Medium, slightly large or larger culets quickly limit the cut grade because they can create visible windows in the table.

The calculator describes its interpretation of girdle and culet choices in a dedicated summary so you can see how they influence the final result.

Warnings For Fisheye, Nail-Head And Extreme Proportions

Beyond simple grading, the calculator looks for proportion combinations that are known to cause visual issues. These warnings are based on geometric risk patterns rather than direct observation.

• Fisheye risk is associated with shallow stones that have low depth percentages combined with overly large tables. This combination can cause the girdle reflection to appear under the table, creating a hazy, fisheye-like look.
• Nail-head risk occurs when stones are too deep with smaller tables. Light can concentrate in the center and turn the middle of the diamond dark.
• Steep-deep combinations, where both crown angle and pavilion angle are high, can also trigger light leakage warnings.

When your inputs trigger one of these patterns, the warnings card explains the possible visual effect so you can ask to see the stone in different lighting or request imaging such as ideal-scope or ASET.

Symmetry And Polish Considerations

The calculator includes a simple dropdown for symmetry and polish, grouped together as Excellent, Very Good or Good. In actual grading, symmetry and polish are separate factors, but they often track together. In this model:

• Excellent symmetry and polish allow the proportions to define the cut grade without further limitation.
• Very Good symmetry and polish may lightly cap a stone that would otherwise be Excellent on proportions alone, depending on the combination.
• Good symmetry and polish can cap the overall result at Good, even if proportions are stronger.

This mirrors the idea that even perfect proportions cannot fully compensate for visibly uneven facets or surface defects.

How To Use The Diamond Cut Grade Proportion Calculator

• Take a laboratory report for a round brilliant diamond and locate the table %, depth %, crown angle, pavilion angle, girdle description and culet size.
• Enter those values into the calculator, along with carat weight if you wish to see it reflected in the summary text.
• Select the symmetry and polish grade bracket that most closely matches the report.
• Run the estimate and note the overall cut grade, the proportion subgrades and any warnings that appear.
• Compare several diamonds by adjusting the measurements and seeing which stones have the strongest geometric support for brilliance.

This process does not replace professional advice, but it gives you a structured way to interpret what the numbers on a grading report mean for real-world performance.