Light bulb fittings explained

Different types of lamp sockets (fittings)

When you buy a lamp or a light source, it’s essential to pick the correct socket/base (also called a fitting). Some types look alike, which can make selection tricky. Below you’ll find a clear overview of the most common fittings and where they’re typically used.

E27 socket

The E27 is the large Edison screw base (“large screw fitting”). The diameter is 27 mm. You simply screw the bulb into the luminaire.

E14 socket

The E14 is the small Edison screw base (“small screw fitting”). The diameter is 14 mm. You screw the bulb directly into the luminaire.

GU10 socket

GU10 is a bayonet-style twist-lock base with pins spaced 10 mm apart. Insert the bulb’s pins, then twist a quarter turn clockwise to lock it in place.

The most common GU10-compatible lamp shapes are MR11, MR16 and AR111 LED lamps. The difference is the lamp diameter:

- MR11: approx. 35 mm diameter
- MR16: approx. 50 mm diameter
- AR111: approx. 111 mm diameter

GU5.3 socket

GU5.3 is a bi-pin push-fit base with two pins spaced 5.3 mm apart. The most common lamp with a GU5.3 base is the MR16 (approx. 50 mm diameter).

GU4 socket

GU4 is a bi-pin push-fit base with two pins spaced 4 mm apart. The most common lamp with a GU4 base is the MR11 (approx. 35 mm diameter).

G13 socket

G13 is the classic base for fluorescent tubes (TL).

G9 socket

G9 is a push-fit base with two looped wide pins and a gap in the middle. The pin spacing is 9 mm.

G5.3 socket

The G5.3 socket has contact “legs” on the outside of the light source. The spacing between these connectors is 53 mm.

G4 socket

G4 is a small bi-pin push-fit base with two pins spaced 4 mm apart. G4 lamps are mainly used in compact luminaires.

R7s socket

R7s is designed for linear (stick) lamps and has contacts at both ends.

Which fitting is used where?