Diamond Table Percentage Calculator

What Diamond Table Percentage Measures

Diamond table percentage (often written as “table %”) describes how wide the table facet is compared with the diamond’s overall face-up width. The table is the large, flat facet on top of the diamond. Because it sits at the center of what you see when the diamond is face-up, table size influences the diamond’s “window” into the stone and can affect how the balance of brightness, fire, and patterning appears.

Table % is always a ratio. It is not a direct measurement in millimeters on its own. A table that looks “large” on a small diamond might be the same table % as a table that looks “normal” on a larger diamond. That’s why table percentage is used: it lets you compare stones of different sizes on the same scale. This calculator helps you convert between table millimeters and table percentage so you can interpret grading reports, compare diamonds quickly, and estimate the physical table size from proportions.

Table % vs Table Size in Millimeters

It is common to see table % on a diamond grading report, but you may also have the table size in millimeters from a measurement, a jeweler’s notes, or an assessment. The relationship is straightforward once you decide what “overall width” means for the stone’s shape.

• Round diamonds: the reference is typically the average diameter at the girdle. Many reports list measurements like 6.45 – 6.48 × 3.98 mm. The average diameter is (6.45 + 6.48) ÷ 2.
• Fancy shapes: a practical consistent reference is the average of length and width: (length + width) ÷ 2. This creates a single comparable “face-up size” number for table % calculations.

Once you have a reference width, table % is simply (table size ÷ reference width) × 100. Conversely, table size is (table % ÷ 100) × reference width. This tool includes both directions so you can start with whichever information you have.

Why Table Percentage Matters for Appearance

Table % is not a “good or bad” number by itself, but it does influence the look of a diamond in predictable ways. A smaller table can emphasize crown height and can sometimes increase perceived fire (colored flashes) when other proportions are supportive. A larger table can increase the amount of light entering and exiting near the center and may produce a different balance of brightness and fire. The key point is that table % is only one lever in a system.

In round brilliants, table % is strongly connected to how the crown and pavilion are proportioned. If the table gets larger while everything else stays the same, crown height tends to shrink. If the table is smaller, crown height tends to grow. Those relationships affect how the stone handles light and how the patterning appears under movement. This is why two diamonds with the same table % can still look different: the “rest of the system” may not match.

How to Use the Table % Calculator Tab

The Table % Calculator tab is designed for the most common real-world scenario: you have measurements in millimeters and want to calculate the table percentage. Start by choosing whether your stone is round or a fancy shape.

• If round: enter the minimum and maximum diameter (mm). The tool uses the average as the reference width.
• If fancy: enter length and width (mm). The tool uses the average of length and width as the reference width.
• Enter the table size in millimeters (table width/diameter).

The result shows the reference width, the calculated table %, an approximate table area estimate, and a small range output you can widen or tighten using the range control. The range is useful when your table measurement is approximate or rounded.

Why the Calculator Shows an Approximate Table Area

Table % is a line measurement: it compares widths. But many people think visually in terms of “surface.” The table is a surface area. Two diamonds with different outlines can have similar table % but different table surface areas because the overall face-up size differs. That’s why the calculator includes a table area estimate:

• Round: area is estimated as a circle using the table diameter.
• Fancy: the tool can estimate table length and width by scaling the outline and then estimate area as an ellipse.

This is an approximation, but it is useful for intuition and lot-level comparisons. It can help you understand why a table feels visually large or small even when table % looks similar across different stones.

Using the Table Size Tab to Convert Table % into Millimeters

The Table Size (mm) tab is the reverse calculation: you know the table percentage and want an estimated table size in millimeters. This is especially helpful when a report lists table % but you want to visualize the physical size, or when you are comparing two stones where one seller provides table % and another provides table width.

For fancy shapes, you also have an optional mode that estimates table length and width by scaling the outline. It assumes the table is proportional to the outline in both directions. While real facet patterns can vary, this proportional scaling provides a consistent estimate that is useful for planning, modeling, and quick comparisons.

Table % and Depth % Are a Pair, Not Competitors

Table % is often discussed alongside depth %. Depth % compares the diamond’s total height to its width. In practical screening, table and depth are a quick way to spot proportions that may push the diamond toward a particular look: a shallow stone can spread larger face-up but may risk light leakage if other angles are not supportive; a deep stone may face up smaller for its weight and can also change the pattern and brightness.

That said, depth % alone is also not enough. The crown and pavilion angles determine how light is bent and reflected. Depth % is a summary measure; angles describe the geometry more directly. This tool’s Proportion Insights tab exists to connect table and depth to the angle story in a simple, readable way.

Proportion Insights: A Practical Screening Lens

The Proportion Insights tab is not a grading engine. It is a “sanity-check lens” that turns numbers into plain-English signals. It looks at table % and depth %, and for round brilliants it can also evaluate crown and pavilion angles. The output is designed to help you ask better questions:

• Is the table unusually small or unusually large compared to common modern ranges?
• Is the depth unusually shallow or unusually deep, suggesting spread or face-up size tradeoffs?
• Do the crown and pavilion angles look broadly balanced, or do they hint at risk zones that deserve closer checking?
• Do finish grades like symmetry and polish support the expectation of crisp patterning and clean reflections?

If you are comparing diamonds online, this tab can help you quickly narrow down candidates worth deeper evaluation. If you are buying in person, it helps you interpret the report numbers and connect them to what you see under different lighting.

Why “Ideal” Numbers Are Not Universal

You will often see charts online that define “ideal” table % and depth % ranges. These can be helpful, but they can also oversimplify. Different cut philosophies prioritize different looks. Some people prefer a brighter, whiter “headlight” look; others prefer stronger colored flashes; others care most about pattern and contrast. A table % that supports one look might not be the best match for another.

The most reliable approach is to use table % as a filter, not as a final decision. Use it to avoid extremes you don’t want, then evaluate the full proportion set, the cut grade, and ideally real performance images when available. And if you’re choosing between two stones with similar lab grades, the small proportion differences can still show up in the “personality” of the diamond.

Lot / Batch Totals for Inventory and Consistency

If you work with multiple stones, table % becomes a useful consistency metric. The Lot / Batch Totals tab estimates an average table size (mm) for each stone based on an average size and an average table %. It also estimates the table surface area and totals it across the batch. This can help with:

• Inventory planning: quickly understand whether a parcel trends toward smaller or larger tables.
• Consistency checks: spot batches that may mix very different looks.
• Communication: summarize proportions in a way that’s easy to compare across lots.

The variability allowance expands totals to reflect real-world spread in measurements. If your lot has mixed sizes or mixed shapes, split it into more consistent groups for better accuracy.

Common Mistakes When Reading Table %

• Mixing units: table % is unitless. If you calculate table size in mm, ensure the reference width is also in mm.
• Using only one diameter for round: round diamonds are rarely perfectly round. Use the average of min and max diameter.
• Judging cut from table % alone: table is one number. The geometry that creates performance is multi-factor.
• Ignoring finish: symmetry and polish don’t replace proportions, but they can affect crispness and the quality of reflections.
• Comparing fancy shapes as if they were all the same: fancy shapes have broader proportion diversity and style differences that matter.

This calculator is designed to avoid the biggest structural mistake: inconsistent references. By choosing a shape reference mode first, you ensure the table calculation is comparable across stones and across reports.

How to Use This Calculator When Buying a Diamond

If you’re shopping, start by gathering the report measurements and table %. Use the Table Size tab if you want to visualize the table in millimeters. Then use Proportion Insights as a quick filter: you’re not trying to “grade” the diamond; you’re trying to identify whether the numbers sit in a reasonable zone for your preferences and whether anything suggests you should dig deeper.

After screening, the best next steps depend on what you have access to:

• If performance images (ASET/IdealScope) are available, use them to validate light return and leakage patterns.
• If you can view in person, compare under multiple lighting environments and observe sparkle patterns, not just brightness.
• When comparing similarly graded diamonds, use proportion differences as a way to predict which stone is more likely to match your preferred look.

The most important takeaway is that table % is a helpful number when it is used correctly: as part of an integrated proportion view and as a consistent comparison metric across stones.