Diamond Proportions: Table, Depth and Why They Change the Sparkle
Behind every cut grade are a few numbers: table %, depth % and two angles. What they mean, the ranges that usually perform well, and how to use them for fancy shapes.
A cut grade sums up a diamond's proportions in one word. Behind it are a handful of measurements printed on every grading report. Understanding them helps you choose fancy shapes, which usually have no overall cut grade, and spot stones that will look bigger or brighter.
Table percentage
The table is the large flat facet on top. Table % is its width divided by the diamond's width. A larger table can mean more brightness (white light); a smaller one can mean more fire (colour flashes). Extremes in either direction usually perform worse.
Depth percentage
Depth % is the total depth divided by the width. Too shallow, and light leaks out of the bottom (a glassy "fisheye" look); too deep, and the stone looks smaller and darker for its weight, because weight is hidden underneath.
Crown and pavilion angles
For round brilliants, the pavilion angle (around 40.6–41°) and crown angle (around 33–35°) do most of the work of returning light. Labs combine these with other measurements to give the cut grade.
Girdle and culet
A medium to slightly thick girdle protects the edge from chipping; a very thin one is fragile, a very thick one hides weight. Most modern diamonds have no culet, or a very small one.
How proportions change the size you see
Two diamonds of the same weight can look different in size. A well-proportioned 1 carat round brilliant measures about 6.4 mm across. A deep 1 carat stone, with weight hidden in its pavilion, may measure only around 6.1 to 6.2 mm, about the same as a well-cut 0.90 carat. This face-up size relative to weight is called the stone's spread, and it is why millimetres on the report matter as much as carats.
Length-to-width ratios for fancy shapes
For elongated shapes, the ratio of length to width decides the outline. There is no single right answer, but these ranges are the most popular:
| Shape | Popular ratio | Look |
|---|---|---|
| Oval | 1.30–1.50 | Classic to slender |
| Pear | 1.45–1.75 | Balanced teardrop |
| Marquise | 1.75–2.25 | Long and elegant |
| Emerald | 1.30–1.50 | Rectangular |
| Cushion | 1.00–1.10 or 1.15–1.30 | Square or elongated |
| Radiant | 1.00–1.05 or 1.20–1.35 | Square or rectangular |
A longer ratio makes a stone look larger and fingers look slimmer; a shorter ratio looks softer and more traditional.
Polish and symmetry
Every report also grades polish (how smooth the facets are) and symmetry (how precisely they line up). Excellent or very good in both is ideal. A "good" grade will not be visible to the eye in most stones, but on a round brilliant it can prevent an excellent overall cut grade, so it is worth knowing what you are paying for.
Well-cut rounds
What is the best depth percentage for a round diamond?
Roughly 59 to 62.5%, with a table of about 54 to 58%, though angles matter too.
Does a big table make a diamond look bigger?
A little, but very large tables can reduce fire. Depth is the bigger factor in face-up size.
Why don't fancy shapes have a cut grade?
Their ideal proportions vary with shape and taste, so most labs grade polish and symmetry instead and leave cut to judgement.
What is a good length-to-width ratio for an oval?
Most people prefer 1.30 to 1.50. Lower looks rounder and softer; higher looks longer and slimmer.
What does spread mean?
How large a diamond looks face-up for its weight. A stone with good spread shows more diameter per carat.
