Screw in bulbs use a base called an Edison Screw or ES base. This base was developed by Thomas Edison for the first light bulbs and is still in use today

Light bulbs come in a variety of shapes and sizes and are generally categorized by their bases (the part that screws into a light socket). The letter E before a number indicates it’s an Edison bulb.


There are four commonly used thread size groups for lamps:

  1. E26 is the size of most light bulbs used in the U.S. It’s referred to as having a “medium” or “standard” base.
  2. E12 is the smaller “candelabra” base. It’s used for nightlight bulbs, and sometimes for decorative light bulbs used in chandeliers and over bathroom mirrors.
  3. E17, “intermediate” base, is in between these two sizes. It’s sometimes used for desk lamps and appliance bulbs, but is not as common.
  4. E39 “mogul” base is used on street lights, and high-wattage lamps (such as a 100/200/300 Watt three-way). Chances are you won’t have a need for these guys.

The number following the E indicates the size in mm of the external thread screw. Thus a E26 has a 26 mm base diameter.

You may see low cost LED bulbs using an E27 base on eBay and Amazon from foreign suppliers. While these technically will work in E26 bases, this is usually a sure indication that the bulb was not strictly designed for the US market and this bulb may lack a proper North American, UL or ETL safety certification and therefore should be avoided.



A Shaped Light Bulbs

“A” type bulb shapes are standard house hold light bulbs. The number after the bulb shape is the number of eighths of an inch in diameter. For metric measured bulbs the number following the bulb shape is the number of millimeters at the widest point of the bulb. Standard bulb sizes include A19 for imperial measurements