What Light Bulb Fitting Do I Need? A Complete Guide to Base Caps and Lamp Holders
Choosing the correct light bulb fitting is essential when replacing a lamp, upgrading to LED lighting, or installing a new fixture. Whether your light uses a B22 Bayonet Cap, E27 Edison Screw, GU10 spotlight fitting, G9 capsule lamp or G4 bulb, identifying the correct bulb base ensures compatibility with your light fitting, lamp holder or socket. This guide explains how to identify common UK and EU light bulb fittings and choose the right replacement bulb with confidence.
Quick Answer: How Do I Know What Light Bulb Base Cap I Need?
The easiest way to identify a light bulb fitting is to check the label on the light fitting. Common fitting codes include B22, E27, E14, GU10, G9 and G4. If the code is missing, examine how the bulb connects to the fitting—whether it twists, screws or pushes into place—and measure the diameter or pin spacing of the base.
In most UK homes, you'll typically find either a B22 Bayonet Cap or an E27 Edison Screw fitting.
What Is a Light Bulb Base Cap?
A light bulb base cap (also known as a lamp cap, bulb fitting or bulb socket) is the connection point between the bulb and the light fixture. It allows electricity to pass from the fitting into the bulb.
Different light fittings require different base caps, which means choosing the correct fitting is just as important as selecting the right brightness, colour temperature, or bulb shape.
Light Bulb Fittings vs Light Bulb Shapes
A bulb shape and a bulb fitting are not the same thing. The fitting (base cap) determines how the bulb connects to the light fixture, while the bulb shape describes the appearance of the lamp.
For example:
- A60 GLS + E27 Edison Screw
- ST64 Filament Lamp + B22 Bayonet Cap
- Candle Lamp + E14 Small Edison Screw
- GU10 Spotlight Lamp + GU10 Base
Always match the fitting first, then choose the preferred bulb shape, brightness and colour temperature.
Most Common Light Bulb Base Types in the UK
B22 Bayonet Cap (BC)
The B22 Bayonet Cap remains one of the most popular light bulb fittings in UK homes.
How to identify a B22 bulb:
- Two metal pins on opposite sides of the base.
- Push-and-twist installation.
- 22mm diameter base.
Common locations:
- Ceiling lights
- Table lamps
- Floor lamps
- Traditional lighting fixtures
E27 Edison Screw (ES)
The E27 Edison Screw fitting is one of the most widely used bulb bases worldwide.
How to identify an E27 bulb:
- Threaded screw base.
- Screws clockwise into the fitting.
- 27mm diameter.
Common locations:
- Pendant lights
- Commercial lighting
- Decorative fixtures
- LED filament bulbs
E14 Small Edison Screw (SES)
The E14 fitting is a smaller screw-type bulb commonly used in decorative lighting.
How to identify an E14 bulb:
- Narrow threaded metal base.
- 14mm diameter.
Common locations:
- Chandeliers
- Wall lights
- Decorative lamps
- Appliance lighting
GU10 Spotlight Fitting
GU10 bulbs are commonly used for directional lighting and LED spotlights.
How to identify a GU10 bulb:
- Two short pins with enlarged ends.
- Push and twist installation.
- Usually found in spotlight fittings.
Common locations:
- Kitchens
- Bathrooms
- Retail displays
- Downlights
G9 Capsule Fitting
G9 bulbs feature a compact design with two looped pins.
How to identify a G9 bulb:
- Two wire-loop pins.
- Small capsule-shaped bulb.
Common locations:
- Decorative fixtures
- Wall sconces
- Under-cabinet lighting
- Modern light fittings
G4 Capsule Bulb Fitting
The G4 fitting is a compact two-pin bulb base commonly used in decorative and low-voltage lighting applications. Modern LED G4 bulbs are increasingly replacing traditional halogen versions due to their improved energy efficiency and longer lifespan.
How to identify a G4 bulb:
- Two straight pins protruding from the base.
- Pins are spaced 4mm apart (hence the name G4).
- Small capsule-shaped bulb design.
- Typically operates on 12V low-voltage systems.
Common locations:
- Under-cabinet lighting
- Display cabinets
- Decorative fittings
- Landscape and garden lighting
- Caravan and motorhome lighting
- Small pendant lights
Important note: Before replacing a G4 bulb, check whether your fitting requires a 12V low-voltage bulb or a mains-voltage alternative. Always match the voltage specifications of your existing lighting system.
Frequently Asked Questions About Light Bulb Fittings
What is the most common light bulb fitting in the UK?
B22 Bayonet Cap and E27 Edison Screw are the most common household light bulb fittings in the UK.
Is B22 the same as E27?
No. B22 is a bayonet fitting that twists into place, while E27 is a threaded Edison Screw fitting.
How do I tell if my bulb is B22 or E27?
A B22 bulb has two side pins and twists into place. An E27 bulb has a threaded screw base that screws into the fitting.
Can I replace a B22 bulb with an E27 bulb?
Not directly. The fitting types are different. You may be able to use a compatible adaptor, but using the correct bulb fitting is always recommended.
What happens if I use the wrong bulb fitting?
A bulb with the wrong fitting will not connect properly to the light fixture and should never be forced into the socket.
Does LED change the bulb fitting?
No. LED refers to the lighting technology, not the base cap. LED bulbs are available in B22, E27, E14, GU10, G9, and many other fitting types.
What does E27 mean on a light bulb?
"E" stands for Edison Screw, and "27" refers to the diameter of the base in millimeters.
Expert Advice from Zico Lighting
With dozens of bulb fittings, lamp shapes and LED technologies available, selecting the correct replacement lamp can sometimes be confusing. At Zico Lighting, we help homeowners, electricians, interior designers and commercial specifiers identify compatible light bulbs every day. If you're unsure whether you need a B22, E27, E14, GU10, G9 or G4 fitting, our team can help verify the correct lamp from a photograph of your existing bulb, fitting or light fixture.