The expression length x width records the two principal outline dimensions of a faceted gemstone, measured at the girdle plane in millimetres. It is one of the standard fields on a laboratory report, an invoice, an appraisal and a memo, and together with the depth measurement (length x width x depth) it defines the overall size envelope of the stone.

Conventions: length is the longer of the two outline dimensions, width is the shorter; for round-brilliant diamonds the two values should be nearly equal and any deviation greater than a few hundredths of a millimetre is reported as a measurement of out-of-roundness. The values are taken with a digital gauge or, on a laboratory report, with optical scanning equipment such as the Sarine, Helium or OGI systems, and are typically rounded to two decimal places.

For fancy shapes, length and width drive the length-to-width ratio (L:W), the principal outline-proportion descriptor. Length x width is also the basis for estimating carat weight from millimetre dimensions when a stone is mounted and cannot be weighed directly; standard formulae and weight-estimation tables (Leveridge gauge, Howard formula, Tag Heuer rules) use the two girdle dimensions together with depth to approximate weight within a few percent, depending on cut style and depth proportions.