Diamond Cut Proportions Calculator

Why Diamond Cut Proportions Matter More Than Most Buyers Expect

When people talk about a diamond “sparkling,” they’re really describing light performance: how efficiently the diamond returns light to the viewer and how that light breaks into bright flashes, colored fire, and crisp contrast patterns. Diamond cut proportions are the measurements and angles that shape that behavior. Color and clarity matter, but proportions often decide whether a diamond looks lively and bright or dull and flat.

A Diamond Cut Proportions Calculator takes the numbers from a grading report or online listing and turns them into a consistent, comparable view. Instead of trying to guess what “56% table” or “61.5% depth” means, you can calculate reference values, identify extremes, and compare two stones side-by-side. The point is not to replace a lab cut grade or expert evaluation. The point is to reduce surprises and help you shortlist candidates that are more likely to look great in real life.

Cut, Shape, and Finish: Three Concepts That Get Mixed Up

It’s common to hear “cut” used casually to mean “shape,” but in diamond grading, these are different:

• Shape is the outline: round, oval, cushion, emerald cut, pear, and so on.
• Cut proportions are the geometry: table %, depth %, crown angle, pavilion angle, and related facet measurements.
• Finish includes symmetry and polish, which influence how crisp the pattern appears and how clean the reflections look.

A proportionally strong diamond with poor symmetry can still lose some “snap.” A symmetrical diamond with risky proportions can leak light. This tool is built around the proportions layer: it helps you understand whether the geometry is broadly balanced before you invest time in deeper evaluation.

The Two Big Ratios: Table % and Depth %

Two numbers show up on almost every grading report: table % and depth %. They’re popular because they summarize a lot of geometry quickly. They’re also easy to misuse if you treat them as magic numbers. The right way to think about them is:

• Table % describes how wide the top table facet is compared with the diamond’s face-up width.
• Depth % describes how tall the diamond is compared with the same face-up width.

For round diamonds, that face-up width is usually the average of the minimum and maximum diameter. For fancy shapes, there isn’t a single diameter, so this calculator uses a consistent reference: the average of length and width. That approach makes comparisons clearer when you’re screening multiple fancy stones from different sellers.

What Crown Angle and Pavilion Angle Actually Do

If table % and depth % are the summary, crown and pavilion angles are closer to the mechanism. These angles describe the slope of the crown facets above the girdle and the pavilion facets below the girdle. In a simplified way:

• Crown angle strongly influences how fire appears (colored flashes) and how the top of the diamond “frames” light return.
• Pavilion angle strongly influences whether light returns to the viewer or leaks through the bottom.

The key is that angles work as a pair. A slightly steeper pavilion might still work if the crown is paired correctly and the table/depth are supportive. Conversely, a “good” crown angle can be undermined by a pavilion that sends light out of the stone. That’s why the Round Brilliant tab includes an “Angle Balance Signal” and why the Compare tab considers combinations rather than a single number.

Girdle and Culet: Small Details With Real Consequences

Girdle thickness and culet size are sometimes treated as minor notes, but they can matter:

• Very thin girdles can be more vulnerable to chipping, especially at points and corners in fancy shapes.
• Very thick girdles can add hidden weight and make a diamond face up smaller for its carat weight.
• Culet size can influence how the very center appears. Larger culets may be visible in certain lighting and can change the look of the center.

This calculator doesn’t try to “grade” these features. Instead, it surfaces practical notes so you remember to check durability and face-up size efficiency when you’re comparing stones.

Star and Lower-Half Facets: The Pattern and “Personality” Layer

Many buyers discover a surprising truth after comparing multiple round brilliants: two diamonds with similar table % and depth % can still look different in sparkle style. Star facet length and lower-half length are part of why. They affect the internal patterning, the width and character of the arrows pattern in hearts-and-arrows style stones, and the balance between broad flashes and smaller, faster scintillation.

In practical terms, you can think of these as “pattern tuning.” The calculator uses these inputs to provide a simple pattern note, not a hard score. If you care about the look, you still want video, clear photos, and ideally performance imagery when available. But as a screening tool, facet length numbers can help you anticipate whether a diamond might lean toward chunkier flashes or a more splintery, glittery look.

Spread: Why Some Diamonds Look Smaller Than Their Carat Weight

Spread is the face-up size relative to weight. Depth % is a strong predictor: deep diamonds often hide more weight below the girdle, which doesn’t add much visible size. If you’ve ever seen a diamond that seems “small for its carat,” depth is often the reason. Spread isn’t the only priority, but it’s a real value consideration. If you’re comparing two stones with similar price and carat weight, the one with better spread can look noticeably larger on the finger.

The Round and Fancy tabs estimate a spread signal from depth % and reference width. It’s a quick way to avoid overpaying for hidden weight, especially when shopping online where you can’t immediately see size differences side-by-side.

How to Use the Round Brilliant Tab

Start with the measurements line from a grading report. Enter the minimum and maximum diameter and the total depth in millimeters. Then enter either table size in millimeters or table percentage, depending on what you have. Add crown and pavilion angles if available, then optionally add star and lower-half percentages.

The output includes:

• Calculated table % and depth % based on consistent reference diameter.
• A practical angle balance signal to flag combinations that deserve closer checking.
• Simple geometry estimates for crown height and pavilion depth to improve intuition.
• A screening score that helps you compare candidates consistently.

If the calculated numbers differ from what you see on a report, that’s a signal that an input is wrong (for example, mixing table mm with table % or using a single diameter instead of average diameter). This tool is also useful as a sanity check for listing data that looks inconsistent.

How to Use the Fancy Shapes Tab Without Overtrusting It

Fancy shapes are more complex because there is more design variety. An oval can show a bow-tie pattern; a cushion can be chunky or crushed-ice; an emerald cut is a step cut with different priorities than a brilliant. Proportions still matter, but the best fancy-shape decision often comes from images and video.

The Fancy Shapes tab focuses on what proportions can do well:

• Compute length-to-width ratio to confirm the outline style you prefer.
• Compute depth % as a spread indicator and to avoid obvious extremes.
• Compute table % consistently and surface caution notes when it’s very small or very large.

Use the “Priority” selector to shape the guidance message. If you care about maximum spread, you’ll screen more aggressively against depth extremes. If you care about a lower profile setting, depth in millimeters becomes more important than depth % alone.

Why a “Score” Is Useful Even If It Isn’t a Cut Grade

Buyers often have a spreadsheet problem: you might be comparing 10–30 listings and you need a consistent method to narrow down. A screening score helps you do that. It’s not a lab grade, and it won’t predict the exact sparkle pattern, but it can quickly flag proportion sets that are more likely to be balanced versus those that are likely to have tradeoffs (like reduced spread, increased leakage risk, or a look that some buyers dislike).

The score in this tool is designed to be conservative and transparent: it rewards being in broadly common balanced zones and penalizes extremes. If you find two stones with similar scores, that’s a good sign: you should let the final decision be driven by imagery, cut grade, and what looks best to your eye.

Comparing Two Stones: What to Look At First

The Score & Compare tab lets you enter two proportion sets and choose a preference. The tool then highlights which set aligns better with that preference and shows key differences and risk flags. In practice, a good comparison order looks like this:

• Check depth % to understand spread tradeoffs.
• Check table % to understand the top-opening style.
• For rounds, check crown and pavilion angle pairing.
• Then validate with images/video and any available performance imagery.

The tool’s recommendation message is intentionally modest. It’s there to help you shortlist, not to “pick a winner” without seeing the stone.

Batch Checking for Traders and Inventory Planning

If you are dealing with multiple stones at once, lot-level screening becomes useful. A batch might trend deeper than you prefer, or it might show wide variability that suggests inconsistent appearance across the parcel. The Lot / Batch Checker estimates table and depth in millimeters from average percentages and a reference width, then totals those values with a variability allowance.

The goal is quick visibility: if a lot is trending deep, you may want to examine face-up size more carefully. If a lot is trending toward extreme tables, you may want to review crown geometry or finish more closely. For final decisions, always evaluate individual stones.

Limits of Proportion-Only Evaluation

A diamond can have “good” proportions and still underperform if optical symmetry is poor or if the stone has light leakage due to subtleties in facet indexing. Likewise, a diamond with slightly unusual proportions can still look great and match a specific preference. Proportions are a map, not the territory.

The most reliable workflow is:

• Use a calculator to screen for extremes and confirm internal consistency.
• Use the grading report to confirm cut grade, symmetry, and polish.
• Use videos and face-up images to evaluate real sparkle, contrast, and pattern.
• If available, use performance imagery (ASET/IdealScope) for deeper confidence.

If you follow that workflow, the calculator becomes a time-saver and a risk-reducer. It helps you focus attention where it matters.