Hypodermic needles are available in a wide variety of outer diameters described by gauge numbers. Smaller gauge numbers indicate larger outer diameters. Inner diameter depends on both gauge and wall thickness. The following chart shows nominal inner diameter and wall thickness for regular-wall needles. Thin-wall needles (not shown) have identical outer diameters but larger inner diameters for a given gauge. Needle wire gauge is derived from the Birmingham Wire Gauge. #1 = 18½ B.W.G.; #2 = 19 B.W.G., and so on to #14 = 31 B.W.G.