How to Identify What Light Bulb Base Cap You Need: A Complete Guide
Wondering what light bulb fitting you need? Whether you're replacing a faulty bulb or upgrading to energy-efficient LED lighting, identifying the correct light bulb base cap is essential. The wrong fitting won't connect to your light fixture, regardless of the bulb's brightness or wattage.
The most common light bulb base caps in the UK are B22 (Bayonet Cap), E27 (Edison Screw), E14 (Small Edison Screw), GU10, and G9. You can usually identify the correct cap by checking the existing bulb, inspecting the light fitting, or measuring the bulb base.
At Zico Lighting, we help homeowners, electricians, facilities managers, and businesses find the right lighting solutions every day. In this guide, we'll explain how to identify your bulb fitting quickly and avoid common mistakes when purchasing replacement bulbs.
Quick Answer: How Do I Know What Light Bulb Base Cap I Need?
If you're unsure what light bulb fitting you have:
- Remove the existing bulb and look for a code printed on the base.
- Check whether the bulb twists, screws, or pushes into the fitting.
- Measure the diameter of the bulb base.
- Compare it with common bulb cap types such as B22, E27, E14, GU10, or G9.
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 or bulb fitting) 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.
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 are the most common light bulb fittings in the UK?
The most common light bulb fittings in the UK are B22 Bayonet Cap, E27 Edison Screw, E14 Small Edison Screw, GU10 spotlight fittings, and G9 capsule fittings.
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.
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 millimetres.
Expert Advice from Zico Lighting
Choosing the correct bulb fitting ensures safe installation, reliable performance, and optimal energy efficiency. If you're unsure what light bulb base cap you need, our team can help identify the correct fitting from a simple photograph of your bulb or fixture.
Whether you're replacing a single lamp or specifying lighting for a commercial project, Zico Lighting provides expert guidance and quality lighting products across the UK and worldwide.