household light bulbs


Most light bulbs screw into a socket. Almost all household bulbs have a medium screw base, sometimes called an edison screw, 1 ¹⁄16 inches in diameter with 7 threads per inch. Bulbs with a base that looks like a medium base but isn't are made to defeat bulb thieves in public places. Some have a left-hand thread; others are the size called “admedium,” used in signs, just a bit bigger (1 532 inches) so they won't fit a residential socket. Other common screw bases, in order of decreasing diameter, are:


Householders usually describe bulb size by wattage, but the industry has a system that reveals the actual dimensions of the bulb.

Bulb shapes are identified by letters: “A” is the shape of a typical household bulb; “B” is the shape of a candelabra base Christmas tree bulb; “C” is the shape of a miniature screw night light bulb, and so on.

The bulb size is then given by a letter for shape followed by the bulb's maximum diameter in eighths of an inch. For example, a T-8 bulb would be a tubular bulb 1 inch in diameter. Ordinary 40-watt and 75-watt bulbs are A-19, with a few A-21.

The size of the base or the shape and size of the bulb are not sure signs of the bulb's wattage or voltage. For example, A-19, medium base bulbs are made for 12 volts, 100 volts, 115 volts, 130 volts and 250 volts.


Collecting old incandescent bulbs is a thriving hobby. A valuable compendium of information on early, pre-standardization sizes can be found in Edward J. Covington's Early Incandescent Lamps at Also, KiloKat's Antique Light Bulb Site at



Sorry. No information on contributors is available for this page.

home | home index | search |  contact drawing of envelope | contributors | 
help | privacy | terms of use