Bulb Size Reference
In most countries of the world the diameter is stated in millimetres. However in the Americas the number represents the maximum diameter in eighths of an inch (e.g. an A19 bulb has a diameter of 19 eighths of an inch, which is 2.375” or 60mm). The imperial measures are sometimes also encountered even in metric countries for certain bulb shapes, particilarly for PAR and MR reflector types.
The most commonly used household bulb is the A-19. The bulb is "A" type and the diameter would be 2 3/8" .