Depth percentage is a calculation used to determine how deep or shallow a diamond's pavilion is relative to the height of the gem. Diamonds with too deep of a depth percentage allow light to "leak" out of the bottom, making the diamond appear dark or dull (these are often called nail heads in the jewelry industry); diamonds with too shallow of a depth percentage allow light to leak out of the sides, robbing the diamond of it's brilliance (these cuts are often called fish-eyes).
To calculate depth percentage, divide the total height of the diamond, from table to culet, by the diamond's total diameter. For example, if a round diamond's total height is 5.6mm and the diameter is 10.0mm, then the depth percentage would equal 56%.
See related: table, table percentage, facets, depth, cut