The base size of a light bulb is measured in millimeters. The diameter is the key measurement for the circular screw base and is best measured using a gauge. With pin type bases, measure the distance in millimeters between the centers of the pins protruding from the base.
Screw bases are identified by a descriptive word, the letter "E" and a number, denoting the diameter. Base sizes range from midget E5 to mogul E40. The standard A-19 light bulb has a medium E26 base. The letter "G" designates pin bases, and additional identifiers specify the number of pins and the distance between their centers. A G5 base indicates a two or bi-pin base with five millimeters between the pins. An "s" designation indicates a single pin, "d" or no designation two pins, "t" three pins and "q" four pins.
The bayonet type base, more common in European fixtures, is placed in the socket with a push fit. Bayonet bases are denoted by BA, followed by diameter in millimeters and an S or D for number of contacts on the base. The wedge base, made by tapering the bulb to seal it, has two wires at the base as contact points.