Light Bulb Base Sizes Explained

Most screw-in bulb codes start with E, for Edison screw, followed by the base diameter in millimeters. E26 means a screw base roughly 26mm across, E12 is roughly 12mm. Once you know that, the numbers stop being arbitrary.

Codes beginning with G are pin fittings rather than screws, and the number again refers to a measurement in millimeters, usually the gap between the pins. GU versions twist and lock into place instead of pushing straight in.

E26 is the standard household base in the United States, used in most table lamps, floor lamps, and ceiling fixtures. E27 is the European equivalent at 27mm and is very close, but they are not officially interchangeable, so buy for your own market.

E12 is the candelabra base found in chandeliers, small decorative lamps, and night lights. E17 intermediate sits between the two and turns up in appliance and some specialty fixtures, and E39 mogul is a large base used in high-output commercial fittings.

GU10 is a twist-lock spotlight fitting with two stubby pins, common in track lighting and recessed downlights. You insert it and turn about a quarter turn to lock. GU24 uses a similar twist action for some ceiling fixtures.

G4, G9, and GU5.3 are small bi-pin fittings used in compact fixtures, under-cabinet lights, and some decorative wall lights. They push straight in and the pin spacing is what distinguishes them, so measure rather than guess.

The simplest method is to read the old bulb, since the base code is usually printed on the metal base or the glass. Failing that, check the lamp itself, as many fixtures print the base type and maximum wattage near the socket.

If nothing is marked, unplug the lamp, let the bulb cool, remove it, and measure the widest part of the base in millimeters. About 26mm is E26, about 12mm is E12. Take the old bulb with you when buying if you are unsure.

Never force a bulb that does not thread or seat easily, since a mismatched base can damage the socket. Base size and voltage are separate matters, so also confirm the bulb is rated for your supply and for the fixture.

Socket adapters that convert one base to another exist, but they change the depth and position of the bulb inside the shade and can push it closer to the shade than intended. Always stay at or below the maximum wattage printed on the fixture.