The names of light bulb shapes are denoted with a letter, which describes the shape, and a number, which indicates the size. The number associated with the shape indicates the diameter of the light bulb at its widest part in eighths of an inch. For example, an A19 light bulb has a maximum diameter of 19/8 of an inch, or 2.375 inches.
![Click to Enlarge Beam Spread Table Chart](/images/bulb-shapes-and-sizes.gif)