What the Depth Percentage on a Diamond Certificate Actually Means
Most buyers look at the 4Cs on a diamond certificate and stop there. But the proportions section — depth percentage, table percentage, and related measurements — contains information that can be more useful than the clarity grade for assessing how a diamond will actually perform. Depth percentage is the one most buyers ask about once they know it exists.
What depth percentage means
Depth percentage is the depth of the diamond (the measurement from the table to the culet, or bottom point) expressed as a percentage of the average girdle diameter.
For a round brilliant with a diameter of 6.5mm and a depth of 4.0mm, the depth percentage is 4.0 ÷ 6.5 = 61.5%.
It tells you, in a single number, how deep the diamond is relative to its width. A diamond that is too deep or too shallow relative to its width will perform differently from a well-proportioned one.
Why depth percentage matters
A diamond that is too deep (high depth percentage) directs light downward through the base rather than reflecting it back up through the table toward the viewer. The diamond looks dark in the centre and has less face-up brilliance. It also faces up smaller than its carat weight suggests, because the weight is concentrated in depth rather than spread across the face.
A diamond that is too shallow (low depth percentage) reflects light straight through rather than bouncing it around the interior facets. The diamond looks glassy and lacks depth of sparkle. It may face up larger than equivalent carat weight in a well-cut stone, but the light performance is compromised.
A well-proportioned diamond has a depth percentage in the sweet spot that maximises internal reflection — light enters through the table, bounces between the pavilion facets, and exits back through the crown toward the viewer's eye. This is what creates brilliance.
Ideal ranges by shape
Round brilliant: 59% to 62.5% is generally considered excellent. Below 58% or above 64% begins to compromise light performance noticeably.
Oval: 58% to 62% is a reasonable target range. Ovals have more natural variation than rounds because there is no standard cut grade, and individual stones vary. A stone at 63% is worth viewing before ruling out; the same at 65% would typically show performance issues.
Cushion: 61% to 67% is the general range for cushion cuts, which have more depth variation by design. Cushion brilliants and cushion modified brilliants have different typical ranges.
Emerald and Asscher: 60% to 67% is typical for step cuts, where depth percentage interacts differently with the open, mirror-like facets.
Depth and the table percentage together
Depth percentage makes most sense alongside table percentage — the width of the flat top facet as a percentage of the diamond's diameter. For round brilliants, a table of 54% to 58% paired with a depth of 59% to 62% is the combination that consistently produces excellent light performance.
These two numbers together give you a much better quick read on a stone's cut quality than either number alone — particularly for fancy shapes where there's no certificate cut grade to reference.
What we look for at Diamond Ateliers
We assess depth and table percentage on every stone we consider, alongside the certificate's other proportion measurements. For clients buying remotely or who want to understand why we recommend one stone over another, these are often the numbers we walk through to explain our selection. Book a consultation to see how they apply to specific stones in person.