How to Use the Light Bulb Socket Guide
Bought a bulb only to find it doesn't twist into your fixture? Light bulb sockets come in several standardized base sizes, each identified by a letter-and-number code that refers to its threaded diameter. Select a socket type above to see its diameter, the fixtures it's commonly found in, and a typical wattage range.
E26, with a base about 26mm across, is the standard household socket in the US and Japan, while E27 (27mm) is the equivalent standard across Europe and much of Asia — the two differ by just 1mm and are generally interchangeable. Smaller E17 sockets show up in mini table lamps and pendant lights, while the even smaller E12 candelabra base is typical of chandeliers and decorative fixtures. GU10 uses a completely different twist-lock mechanism with two pins instead of screw threads, common in recessed and track lighting, so it isn't interchangeable with any Edison screw base. B22 bayonet sockets are the household standard in the UK, Australia, and other Commonwealth countries rather than the US.
Frequently Asked Questions
The diameter differs by only 1mm, so they're physically compatible in most cases. However, since E26 and E27 are standard in different regions, double-check the voltage rating of the bulb before using one interchangeably.
Remove the old bulb and check the base for a printed code like E26 or E17, or measure the socket's threaded diameter with a ruler. Most U.S. household fixtures use E26.