What is diamond depth percentage, and why does it matter?

Depth percentage is the ratio of a diamond's total depth to its girdle diameter, expressed as a percentage on an IGI grading report alongside the actual millimetre measurement from culet to table. The culet is the pointed base of the stone; the table is the flat top surface. Together, these two reference points define the vertical distance that determines how light travels through the diamond. A pavilion cut too shallow or too deep causes light to leak out the sides or bottom rather than reflect back through the crown, which is why two lab-grown diamonds with identical carat weight and colour can look noticeably different in brightness. For a lab-grown round brilliant, the accepted range sits between 59% and 62.5%; go above 64% and brilliance drops noticeably. That figure, however, reads differently across cuts: a lab-grown princess cut at 75% to 78% depth performs well optically, while that same depth on a round would kill its light return. This is why depth percentage cannot be treated as a standalone metric. IGI grading reports include depth percentage alongside table percentage and an overall cut grade, and reading all three together gives a far more reliable picture of optical performance than any single number. A well-proportioned lab-grown diamond at a given carat weight will consistently outperform a poorly proportioned one, regardless of size.