Diamond Depth and Table Percentages, the Numbers Behind the Sparkle

A GIA Excellent grade covers more ground than most buyers realise. Two stones with identical grades can differ by several hundred dollars and, more importantly, by a visible amount of face up size, all because of two numbers most people scroll past on a certificate: depth percentage and table percentage.

Our diamond cut guide explains how the GIA's overall cut grade is built and what it actually measures. This post stays narrower. Just these two proportions, the ideal ranges, what happens physically when a stone sits outside them, and why it matters more than colour or clarity in some buying situations.

Two numbers, one outcome

Table percentage is the width of the flat top facet expressed as a percentage of the stone's average girdle diameter. A round brilliant with a 6.5mm diameter and a table facet measuring 3.6mm wide has a table percentage of 55%. Simple geometry.

Depth percentage is the total height of the stone from table to culet, again divided by the average girdle diameter. A stone 6.5mm across and 3.9mm tall has a depth percentage of 60%.

Both sit on every GIA, AGS, IGI, and HRD certificate. Neither requires anything beyond the numbers printed on the grading report. And yet these two ratios predict optical behaviour more accurately than almost any other measurement outside of a full angular analysis.

The ranges that actually work

For a round brilliant, ideal table percentage sits between 53 and 58 percent. Ideal depth runs from 59 to 62.5 percent.

These aren't conservative guesses or ranges invented by a retailer. They come from optical modelling confirmed by the performance of well-cut stones across decades of grading data. Within those windows, a round brilliant returns light to the viewer in the balance of brightness, fire, and scintillation that defines an excellent cut.

Proportion	Below ideal	Ideal range	Above ideal
Table %	Weak light entry, dark centre	53 to 58%	Light leakage, reduced scintillation
Depth %	Fisheye effect, light exits through base	59 to 62.5%	Weight in pavilion, smaller face up appearance
Crown angle	Low fire, flat look	31.5° to 36.5°	Crown blocks returning light, reduced brightness
Pavilion angle	Nail head shadow visible from above	40.6° to 41.8°	Dark centre, light leaks sideways

A stone doesn't need to be far outside these ranges to show the effects. Even a depth of 63.5% can produce a measurable reduction in face up size. The grading report won't flag it. You have to know to look.

What goes wrong at the edges

A table above 58% sounds like a rounding error. It behaves like more than that. The table facet is where incoming light enters, but the angled star facets and upper girdle facets surrounding the table are what generate fire, those coloured light flashes that separate a great diamond from a merely bright one. Widen the table and you compress those facets. Light enters more directly, bounces fewer times, disperses less. In overhead lighting the stone looks fine. In candlelight or diffuse indoor light it looks flat.

Narrow the table below 53% and the problem reverses. The stone collects less light arriving at an angle. Viewed straight down through the table, the centre looks dark. This failure mode is less common in certified stones because cutters aiming for GIA Excellent know the boundary well, but it does appear in older or unusual cuts marketed as round brilliants.

Shallow stones are the most forgiving looking until you know what the fisheye is. A depth below 59% means the pavilion facets are angled too gently to reflect light back upward. Instead, light passes through the base of the stone and you see a pale, transparent ring through the table. Not a subtle characteristic. Once you've identified it, you can't not see it.

Deep stones have a different failure, and it's the one we see most often across the market. Depth above 62.5% means the stone is carrying extra height in its pavilion that contributes nothing to its face up appearance. The weight is there. You're paying for it. You're just not seeing it.

The face up size problem

This is where depth percentage stops being an optical technicality and becomes a direct buyer concern.

A well-proportioned 1.00ct round brilliant at 60% depth will have an average girdle diameter in the region of 6.4 to 6.5mm. Take the same carat weight and push the depth to 64%, and you're looking at approximately 6.1 to 6.2mm across. A difference of roughly 0.3mm sounds small. On a finger, set in a ring, against the skin, it's visible. The stone looks smaller. Lighter in presence. And because it's the same carat weight, the price is comparable or even higher if the colour and clarity grades are strong.

That's the hidden tax on deep stones. You're paying for carat weight buried in the pavilion doing nothing. The person you're buying this ring for will never see that weight. Neither will anyone looking at the ring.

For buyers who care about face up presence relative to budget, and particularly those considering whether to go up in carat weight, the carat weight guide covers how actual face up size varies across shapes and weights. Within round brilliants specifically, depth percentage has more effect on perceived size than most buyers expect.

Crown and pavilion angles

Depth percentage and table percentage are partly downstream of two more granular measurements: crown angle and pavilion angle.

Crown angle is the angle between the bezel facets and the girdle plane. The ideal range for a round brilliant runs from 31.5 to 36.5 degrees. Below 31.5 degrees, the crown is nearly flat, and the geometry that generates fire disappears. Above 36.5 degrees, the crown starts blocking the light returning from the pavilion, reducing brightness.

Pavilion angle has an even larger effect on optical performance. The target is 40.6 to 41.8 degrees. Within that range, the pavilion facets work as internal mirrors, reflecting light back up through the crown toward the viewer. Drop below 40.6 degrees and a dark circular shadow forms at the centre of the stone when viewed from above. That's the nail head effect, and it appears more often than people expect, particularly in stones at the shallow end of the depth range. Exceed 41.8 degrees and the stone starts leaking light sideways, producing the absorbed, dark look of a cut that runs too deep.

These angles don't always appear in a prominent position on certificates. GIA reports include them in the proportion data but they're not the headline figure. If you have access to a full Sarine report, checking the angles against table and depth percentages tells you whether the proportions are internally consistent. A stone can have a table of 56% and a depth of 61% but still underperform if the crown and pavilion angles sit at opposite extremes of their respective ranges.

Using this at the search stage

The practical application is simple. Use proportion filters before you shortlist stones, not after. Table between 53 and 58%, depth between 59 and 62.5%: apply those filters and you've narrowed down to the highest-performing portion of the GIA Excellent round population. You can run that search across our index of over 14 million stones using the advanced search.

Among GIA Excellent rounds in our index, a meaningful share carry depths above 63% or tables above 58%. They all hold the same grade. Many are priced attractively, and that's partly why they're still on the market. The price gap between an optimally proportioned Excellent and a deep Excellent in the same colour and clarity grade is real but often modest. The face up size difference is also real and rarely discussed.

Deeper stones aren't always the wrong choice. A buyer setting a stone in a deep bezel, where much of the girdle will be concealed, may care less about face up diameter. But a buyer prioritising the visual presence of the stone should care quite a lot.

The numbers that don't move

Colour grades shift with laboratory regrading trends. Clarity grades carry a subjective element that varies between graders. But the physics of a round brilliant's proportions are fixed. A table at 57% and a depth at 61% will behave the same way next year as they do today, regardless of what happens to lab-grown prices or certification trends.

That stability matters. When we look at round brilliants with consistently strong optical performance across retailers and price brackets in our index, proportion consistency is the one thing they share. Not the same retailer. Not the same price band. Just stones sitting inside the same narrow windows that optical science has been pointing to for decades.

The next time you're comparing two GIA Excellent rounds and one appeals to you more without knowing why, check the depth and table figures first. Nine times out of ten, that's where the answer sits.