What Are the Different Types of Light Bulb Fittings

The part of the lamp or light bulb that connects into the light fitting is generally known either as the "cap" or "base".

The cap provides the electrical contact to conduct electricity to the light bulb but it also helps to secure the light bulb into its fitting.

There are a vast variety of caps and bases that exist in order to help make sure that only the correct type of lamp is used in any given fitting. This section shows many of the most popular fittings.

Bayonet Cap



Diameter: 22mm
Diameter (Pin to Pin): 27mm
Height: 26mm

With its familiar "push and twist" action, the "bayonet cap" (also known as BC or B22d cap) is used on most regular light bulbs. It is 22mm in diameter and with two locating lugs.



Diameter: 15mm
Diameter (Pin to Pin): 17mm
Height: 26mm

The "small bayonet cap" (SBC or B15d) is very similar but only 15mm across.

Although generally used for mains voltage lamps, the SBC fitting can also be found in a very small number of specialist low voltage halogen lamps.

There are also many other bayonet cap variants including the 3-pin BC B22d-3 sometimes used on Fireglow lamps but perhaps more commonly on High-pressure mercury lamps for industrial applications. The BY22d is used on some low-pressure sodium (SOX) lamps.

Edison Screw Cap

What is an E27 light bulb?

Named after the pioneering inventor Thomas Edison, the Edison Screw cap or "ES" lamp fitting is used worldwide in a vast range of applications.



Diameter: 27mm
Height: 26mm

The most popular ES or E27 fitting is 27mm diameter and is widely used in UK and Europe. This cap is the standard 27mm diameter screw cap for UK 240V light bulbs - not to be confused with E26, which is a 26mm size and is designed for the 120V US market.



Diameter: 14mm
Height: 26mm

The SES or "Small Edison Screw" cap is often used for smaller decorative fittings, chandeliers, and appliance bulbs - It has a diameter of 14mm and is predominantly used in the UK and Europe.

There are also many size variations, mainly for use in specialist fittings. The MES-E10 fitting is sometimes used in large chandeliers containing perhaps dozens of small lamps.







Lilliput Edison Screw




Miniature Edison Screw




Candelabra Edison Screw




Small Edison Screw




Edison Screw




Giant Edison Screw


Capsule Lamps

Miniature halogen capsule bulbs are generally used in integrated fittings and appliances, such as cooker hoods and kitchen cabinet lights.

Capsule bulbs can be identified by their miniature size and they all have 2-pins extruding out of the base of the capsule. The model of the capsule is designated by the measurement in mm between these two pins.

The different distances between the pins prevent the wrong type of capsule bulb being inserted into the wrong fitting - such as a 12V G4 capsule being inserted into a 240V G9 light fitting.

G9 Capsule Pins


Distance Pin (Centre) to Pin (Centre): 9mm

The G9 is specifically used for 240V capsule bulbs. The bulb can be identified by measuring the distance between the centre points of the pins. The distance of a G9 measures 9mm.

G4 Capsule Pins


Distance Pin (Centre) to Pin (Centre): 4mm

The G4 is specifically used for 12V capsule bulbs. These capsules require a 12V transformer or driver to operate - this is usually built into the fitting. The capsule can be identified by measuring the distance between the centre points of the pins. The distance of a G4 measures 4mm.

GY6.35 Capsule Pins


Distance Pin (Centre) to Pin (Centre): 6.35mm

The GY6.35 is specifically used for 12V/24V capsule bulbs. These capsules require a 12V transformer or driver to operate - this is usually built into the fitting. These capsules are often used for task lighting. The capsule can be identified by measuring the distance between the centre points of the pins. The distance of a GY6.35 measures 6.35mm.

LED Capsule Bulbs are now also available as an energy efficient alternative to halogen bulbs. The fitments on LED capsules are exactly the same as those on halogen capsules, so if you're replacing your current bulbs with LEDs and your fitting accepts one of the standard types listed here, you should be able to install them without an issue.

Linear Halogen

Linear Halogen R7s


Cap Diameter: 7mm
Cap Length: 5mm
Bulb Length: 78mm/117mm

Linear Halogen bulbs for floodlights and up-lighters have a 7mm R7s cap at each end of a long quartz linear bulb. All linear halogen bulbs have the same cap but come in either 78mm lengths or 117mm length bulbs. They also come in a variety of wattages, so it is important that a replacement linear halogen is like-for-like (same wattage, same length).

Halogen and LED Spotlights

The most common halogen spots are either push fit (GU4/MR11 or GU5.3/MR16), low voltage type or twist and lock (GU10 or GZ10) mains 240V versions.

Spotlight bulbs can be identified by measuring the distance in mm between the centre of the two pins that extrude from the base of the spotlight.

The different distances between the pins and the size of the pins prevent the wrong type of spotlight being inserted into the wrong fitting.



Type: Twist and Lock
Distance Pin (Centre) to Pin (Centre): 10mm

Featuring two pins that twist and lock the spotlight into the fitting, the GU10 is the most popular spotlights used in the UK. This spotlight is mains operated 240V and was initially developed as a halogen spotlight but is nor widely available in LED. Various wattages, colours and beam angles can now be purchased to provide lighting in many different applications.



Type: Push Fit
Distance Pin (Centre) to Pin (Centre): 5.3mm

This base of this lamp is technically named GU5.3, however, the whole spotlight is more commonly referred to as an MR16. Featuring two round pins that push into the fitting, this is the 2nd most popular type of spotlight. The MR16 spotlight operates at 12V so requires a 12V transformer or driver to run-  these drivers are sometimes built into the fitting but can also be purchased separately. 


The GZ10 spotlight looks very similar to the GU10 lamp, but there are two major differences. The GZ10 spotlight makes use of a dichroic reflector to direct all of the heat generated by the spotlight backwards, creating a cool beam spotlight. The GZ10 spotlight, therefore, features a square corner base (where the GU10 has a bevelled base). This stops the GZ10 being used in a light fitting designed for a GU10 where this backwards reflected heat could damage the fitting. Note: although a GZ10 cannot be used in a GU10 fitting, a GU10 can be used in either a GU10 or GZ10 fitting.

Fluorescent Tubes and LED Tubes

Fluorescent and LED tubes have a two-pin fitting at both ends of the tube.

T8 Tube G13 Cap


Tube Diameter: 25mm (T8 = 8/8 inch)
Distance Pin (Centre) to Pin (Centre): 13mm

The most common size of fluorescent or LED tube is the T8 which uses a G13 two-pin cap on both ends of the tube. The distance between the pins is 13mm and when inserted into the fitting, the tube twists to lock into place.

This cap is also found on T10 (tube diameter: 10/8 inch) and the larger T12 (tube diameter: 12/8 inch).

T5 Tube G5 Cap


Tube Diameter: 16mm (T8 = 5/8 inch)
Distance Pin (Centre) to Pin (Centre): 5mm

There is a smaller tube size - the T5 which uses G5 two-pin caps. The distance between the pins is 5mm.

This cap can also found on less popular T4 (tube diameter: 4/8 inch).