How to Measure Lamps and Light Bulbs
All lamps have a specific "bulb type" designation that describes the shape of the lamp and the size of the lamp (diameter). The designation is a two part code consisting of a letter and a number. The letter indicates the shape of the lamp and the number relates to the diameter of the lamp in eighths of an inch.
The reference ruler illustrates the most commonly used household lamp, an A19. The "A" indicates the shape (standard) and the diameter would be 23.8", which is "19" eights of an inch.
All lamp types are available in a variety of technologies and therefore may vary in appearance. Maximum overall length (MOL) will also vary.
Additional popular lamp designations are also indicated on the reference ruler.
A: Arbitrary (Standard)
PS: Pear Shape
B/BA: Bulged, Bulged with angular tip
C/CA: Conical, Candel shape with bent tip
RP: Reflector, Pear shape
S: Straight-sided shape (compare with CA and BA)
ST: Spherical Tubular
MR: Multi-Faceted Reflector
BR: Bulbous Reflector
BT: Bulged Tubular
E/ED: Elliptical, Elliptical with dimple in the crown
AR: Aluminized Reflector
PAR: Parabolic Aluminized Reflector