Light bulbs and lighting lamps are usually described by shape, length, diameter, and screw type designation.
Bulbs are also defined by a code number consisting of a letter or letters followed by a number. The letter indicates the shape of the bulb and the number relates to the diameter of the bulb in eighths of an inch.
The most commonly used household bulb is the A-19. The bulb is “A” type and the diameter would be 2 3/8″.
An A-19 is 19 eighths inches or 2 3/8 inches in diameter
A 4′ T12 fluorescent tube is 12 eighths inches or 1 1/2 inches in diameter
A 4′ T8 fluorescent tube is 8 eighths inches or 1 inch in diameter
So if you want to know the actual diameter of the bulb you can usually divide the model number by 8.