What Are the Different Types of Light Bulb Fittings, Caps and Bases?

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

Caps provide 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

BC B22 Bayonet

BC-B22d 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.

See our range of BC-B22 capped lamps.


SBC-B15d Cap

  • 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.

See our range of SBC-B15d capped 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

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

ES E27

ES-E27 Cap

  • 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.

See our range of ES-E27 capped lamps.


SES-E14 Cap

  • Diameter: 14mm
  • Height: 26mm

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

See our range of SES-E14 capped lamps.

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.





E55mmLilliput Edison ScrewLES
E1010mmMiniature Edison ScrewMES
E1212mmCandelabra Edison ScrewCES
E1414mmSmall Edison ScrewSES
E2727mmEdison ScrewES
E4040mmGiant Edison ScrewGES

Capsule Lamps

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

Capsule light 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 light bulb being inserted into the wrong fitting – such as a 12V G4 capsule being inserted into a 240V G9 light fitting.

G9 Base

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

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

See our range of G9 capped lamps.


G4 Base

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

The G4 is specifically used for 12V capsule light 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.

See our range of G4 capped lamps.


GY6.35 Base

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

The GY6.35 is specifically used for 12V/24V capsule light 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.

See our range of GY6.35 capped lamps.

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

Linear Halogen


R7s Fitting

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

Linear Halogen light bulbs for floodlights and up-lighters have a 7mm R7s cap at each end of a long quartz linear light bulb. All linear halogen light bulbs have the same cap but come in either 78mm lengths or 117mm length light 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).

See our range of R7s capped lamps.

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.


GU10 Cap

  • 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.

See our range of GU10 capped lamps.


GU5.3/MR16 Cap

  • 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.

See our range of GU5.3 (MR16) capped lamps.

GZ10 Cap

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.


G13 Fitting

  • 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.

Also found on T10 (tube diameter: 10/8 inch) and the larger T12 (tube diameter: 12/8 inch).

See our range of G13 capped tubes.


G5 Fitting

  • 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).

See our range of G5 capped tubes.

Now that you can identify which cap you need for your light fitting, don’t forget to read our guide on choosing the right LED light bulb for you.

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Hi I have a 4 pin saver bulb fitting 13wdo you have other styles or do you have this in stock please

Mat @ Lightbulbs Direct

There are various types of 13W 4-pin energy savers, so we will require some more information from you – can you see a manufacturer name and product code printed on the bulb?


Hello I have light shades that are shallow and currently they are lit by16W Compact Fluorescent CC, CLC16W2PIN, GR8. Is there an alternative. Hope this information is sufficient for an answer. Thanks.

Mat @ Lightbulbs Direct

Hi Janet, we’ve recently started selling an LED replacement for your 16W CC, our 7W LED DULUX from Ledvance is an LED designed to replace that exact lamp, and can be used directly with your existing fitting.


Hi, table lamp has a large type screw fitting for bulb but it has a central prong about a inch long and 1mm wide. Do you know which light bulb fits.

Mat @ Lightbulbs Direct

Hi Mike, no, cannot think what that could be – do you want to send a photo through to our customer service team and we can see if we can identify it?


Does anybody know where I can buy an E5 candle screw fitting for a chandelier?

Mat @ Lightbulbs Direct

Sorry, John, we don’t stock anything as small as an E5

Chris cadogan

i need to buy an under cabinet replacement bulb but the only identifying mark is WH-03416 and also (2835 0.2W). There is no mark on the transformer box. Can u help?

Mat @ Lightbulbs Direct

Can’t find much information on WH-03416, but 2835 0.2W is a type of LED chip – are your fittings LED and are you sure they have a replacement bulb and the LED is not integrated thus not replaceable?

Melisa Jensen

Hi do B22 bayonet bulbs exist in the US? I sent a lamp to the US with a bayonet cap and a U.S. plug but the customer is struggling to find a bulb that fits the lamp.

Mat @ Lightbulbs Direct

Not widely no, the US tends to favour ES caps. Would a B22 to E27 convert help?


I have a low eneregy ceiling light fitting which is about twenty years old.It is not a bayonet type but the light bulb fits snugly into the fitting . It doesnt seem possible to unscrew the long life bulb and any amount of manoeuvering doesnt seem to release the light bulb from the fitting. Does amyone have any suggestions as to how it can be removed? I would post a picture but that doesnt seem poosible on this message. Thanks.

Mat @ Lightbulbs Direct

Hi Sam, email through to our customer service team enquiries.com with a photo and we will do our best to assist and possibly point you in the direction of a suitable replacement bulb.


Hello, I posted a query earlier about which tube to get. On the box from my old one. It saysF58W/835
5200 Lumen
Do you have this on your site?

Mat @ Lightbulbs Direct

HI Ingrid, the tube you need is a Fluorescent 5ft T8 in white. Unfortunately, we are out of stock of those at the moment (they are due in 21st August), but do have Fluorescent 5ft T8 in cool white versions which are similar. Alternatively search F58W/835 in google and you will find suitable tubes – this code is a standard code used by all manufacturers to categorize that exact tube.

Terry Randall

Hi There
A 12 volt capsule light bulb stopped working in my 20 year old caravan. I replaced it with a new G4 capsule but it does not work. Any suggestions? Could the old capsule be different? The rest of the system seems to be OK. Thanks

Mat @ Lightbulbs Direct

My suggestions, would either be a faulty capsule bulb, or perhaps the fitting it inserts into has corroded in some way?


Can you get some sort of extender for light bulbs? I have a light shade which makes it so the bulb cap that fits through is too short to connect into the socket. Can’t fit the light shade til I find a solution

Mat @ Lightbulbs Direct

Not an extender as such, but we have lightbulb socket converters which extend the length of a bulb when used. If the light fitting is an screw cap, you could buy the screw to bayonet converter, then use a bayonet light bulb.


Hi. I have old spot lights from ikea that use Halogen GU4 bulbs. It has a 12v transformer. Can I change to Led bulbs?

Mat @ Lightbulbs Direct

It does depend on the specifications of the 12V transformer, and whether it would support the low power consumption of LED. Best way to find out, is to simply purchase some LED G4‘s and take the opportunity of our 28-day return policy to test them out (keep the packaging in good condition so you can return them for a refund).

John Hanna

I am looking for an older type 4 pin light bulb

Mat @ Lightbulbs Direct

Hi John, he are our 4-pin bulbs – you will need to make sure you match the exact wattage between the old and new bulb for compatibility.

T m

Why is the “ES-E27 Cap” illustrated with a pic showing 40mm diameter of the screw base ?

Mat @ Lightbulbs Direct

Thanks for spotting that error Tom, we’ve updated the image

Alex Williams

Hello, I have an old lamp, bayonet bulbs needed but the electrical contact on the bottom doesn’t seem to work on new bulb. Old bulb looks different to new bulb bought- the contact area is smaller and therefore doesn’t connect. Any advice – hope that makes sense

Mat @ Lightbulbs Direct

If the bayonet is the same width and type as the old bulb, then my guess would be that the spring loaded connectors in the fitting (that make connection with the electrical contacts) may not be pushing as high as they once did. For electrical safety, I would recommend getting an electrician to install a new bayonet base in the fitting.

Tammy Moore

I have a rectangle socket chandelier and don’t know where to look to find the bulb that goes with it

Janet L Oge

What about an “ST12”? I have yet to find information on this bulb.

Mat @ Lightbulbs Direct

ST12 refers to a squirrel cage shaped light bulb with an E12 cap – generally a very rare cap size mainly used in the US market.

Byron McIntosh

Thanks very clear and easy to understand

Brian Sumner

How can I specify two contact points in lieu of a central single point on a BC22 Bayonet Cap bulb


Why are some bulbs ES and some are SES? Why use a 27mm ES when a 14mm SES would do? All bulbs range from 40W to 100W and you can get 100W SES, so why use ES? What is the reason for the larger screw? I know that industrial high power lights use a E40 screw fitting and that makes sense for the much higher wattage required. It’s like Why use the 2 pin shaver sockets when the Standard 13A 3 pin will provide the same connectivity and be safer to power?

Mat @ Lightbulbs Direct

We would love to know the reason too! Stocking all these different variations of caps for each light bulb takes up a lot of warehouse space!


There are ads for bulbs specifying B14 and B15. What is the b14 or is it an error?

Mat @ Lightbulbs Direct

I would suggest it is an error, we’ve never heard of a B14 – don’t suppose your are seeing ads for E14 – i.e. a 14mm small screw cap?

Wasiq iqbal


Margaret Woodall

do i have a dodgy light fitting or do concrete ceilings have an impact on 4LED G9 Capsules

Mat @ Lightbulbs Direct

I wouldn’t have thought so; LED G4 capsules produce very low heat, so there shouldn’t be a problem dissipating that heat in a concrete ceiling. What are the problems you are experiencing? Anything else in the circuit that could be a contributing factor?


I’m not sure what bulb I need can anyone help?

Mat @ Lightbulbs Direct

Hi Abi, give our customer service team a call on 01494 723 286 we have a wealth of experience trying to identify light bulbs and find replacements.


I’ve bought a lamp today but I’m struggling to find which bulb fits. Can anyone help please?

Mat @ Lightbulbs Direct

Is there is any text written on the lamp; there is usually a cap type and max wattage.?

Dave Morgan

very informative, thank you