Depth percentage

Depth percentage is the total height of a faceted diamond (table to culet) divided by its average girdle diameter, expressed as a percentage. On a round brilliant the GIA Excellent depth window sits at 59.0% to 62.6%; AGS Ideal-0 rounds run 60.5% to 62.5%. Outside roughly 58% to 63.5% the cut grade drops.

Depth percentage matters because it surfaces hidden weight. A diamond's carat number measures mass — it is independent of how that mass is distributed inside the stone. Cutters routinely retain weight from the rough by leaving extra material in the pavilion (deep pavilion, deep girdle, or a combination), which inflates depth percentage and shrinks face-up appearance. A 1.00 ct round with a 64% depth can face up smaller than a 0.92 ct round with a 61% depth — the buyer paying for the larger carat number is paying for diamond that lives below the girdle where nobody sees it.

The other end of the band — shallow depth, under 58% — is the opposite failure: too much spread, too little material to support proper crown and pavilion angles. Shallow rounds face up large for their carat weight but leak light through the pavilion and show a visible window through the table. The cut grade drops because the proportion geometry can no longer return light correctly regardless of how cleanly the facets are polished.

Depth percentage is a check, not a target. A stone with a depth in the GIA Excellent band can still face up small if its girdle is thick, or face up large if its girdle is thin. The diameter measurement on the report (in millimeters) is the actual face-up size signal — depth percentage explains why two stones with the same carat weight have different diameters. For round brilliants the rule of thumb is that 1.00 ct should measure roughly 6.4 mm to 6.5 mm; anything materially under 6.4 mm at 1.00 ct is a high-depth stone trading face-up size for weight.

Fancy shapes behave differently. A princess at 70% depth is normal; an emerald cut at 67% depth is shallow. Cushions tend toward 65% to 70% depth at top cut grades because of the curved silhouette. Comparing depth percentage across shapes without correcting for shape conventions produces wrong conclusions. Within a shape, deeper than the published band is the price-per-carat trap a buyer pays for and never sees.