Guide to Choosing the Correct Lamp Base, Cap, or Lamp Holders

Choosing the correct light bulb fitting is essential when replacing a lamp, upgrading to LED lighting, or installing a new light fixture. Whether you need a B22 Bayonet Cap, E27 Edison Screw, GU10 spotlight, G9 capsule, or G4 lamp, identifying the correct base cap ensures your new bulb is compatible with your existing lamp holder or socket.

This guide explains the most common UK light bulb fittings, how to identify them, and how to choose the right replacement with confidence.

The simplest way to identify a light bulb fitting is to:

  • Check the existing bulb or light fitting for a fitting code such as B22, E27, E14, GU10, G9, or G4.

  • Look at how the bulb connects—does it twist, screw, or push into place?

  • Measure the base diameter or the spacing between the pins if you're unsure.

In most UK homes, the two most common fittings are B22 Bayonet Cap (BC) and E27 Edison Screw (ES).

What Is a Light Bulb Base Cap?

The base cap (also called a lamp cap, bulb fitting, or lamp holder connection) is the part of the bulb that connects to the light fitting and supplies electrical power.

Every light fitting is designed to accept a specific base cap, so 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

Many people confuse the bulb fitting with the bulb shape, but they are two different things.

The fitting determines how the bulb connects to the light fixture, while the shape describes the appearance of the bulb.

Examples

  • A60 GLS bulb with an E27 Edison Screw

  • ST64 Filament bulb with a B22 Bayonet Cap

  • Candle bulb with an E14 Small Edison Screw

  • LED Spotlight with a GU10 fitting

Always choose the correct fitting first, then select the bulb shape, brightness, colour temperature and dimming option that best suits your application.

The Most Common Light Bulb Fittings 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 locating pins on opposite sides

  • Push into the holder and twist to lock

  • 22mm diameter base

Common applications

  • Ceiling lights

  • Table lamps

  • Floor lamps

  • Traditional light fittings

E27 Edison Screw (ES)

The E27 Edison Screw is one of the world's most widely used bulb fittings.

How to identify an E27 bulb

  • Threaded metal screw base

  • Screws clockwise into the holder

  • 27mm diameter

Common applications

  • Pendant lighting

  • Ceiling fixtures

  • Commercial lighting

  • Decorative LED filament lamps

E14 Small Edison Screw (SES)

The E14 Small Edison Screw is commonly used in decorative lighting where a smaller bulb is required.

How to identify an E14 bulb

  • Slim threaded screw base

  • 14mm diameter

Common applications

  • Chandeliers

  • Wall lights

  • Decorative table lamps

  • Appliance lighting

GU10 Spotlight Fitting

The GU10 is the standard fitting for mains-voltage LED spotlights.

How to identify a GU10 bulb

  • Two short pins with enlarged ends

  • Push and twist installation

  • Usually fitted into recessed spotlight holders

Common applications

  • Kitchens

  • Bathrooms

  • Downlights

  • Retail displays

G9 Capsule Fitting

The G9 is a compact mains-voltage capsule lamp used in many modern decorative fittings.

How to identify a G9 bulb

  • Two wire-loop pins

  • Small capsule-shaped lamp

  • Push-fit installation

Common applications

  • Wall lights

  • Decorative fittings

  • Under-cabinet lighting

  • Modern ceiling fixtures

Important: When replacing a G9 halogen lamp with an LED G9 lamp, always check that the LED lamp is suitable for your light fitting. Some LED G9 lamps are not designed for use in enclosed (closed) luminaires, where there is little or no free airflow around the lamp. Using an LED lamp in a fully enclosed fitting when it is not rated for this application can lead to overheating, reduced performance, and a significantly shorter lamp life.

A closed fitting is any luminaire where the lamp is enclosed by a cover, shade or diffuser that restricts the free circulation of air around the lamp. Always refer to the manufacturer's specifications to ensure the LED lamp is approved for use in enclosed fittings.

G4 Capsule Fitting

The G4 is a compact two-pin fitting most commonly found in low-voltage decorative lighting. Modern LED G4 lamps are increasingly replacing halogen versions thanks to their lower energy consumption and significantly longer lifespan.

How to identify a G4 bulb

  • Two straight pins

  • 4mm pin spacing

  • Compact capsule design

  • Usually operates on a 12V low-voltage system

Common applications

  • Under-cabinet lighting

  • Display cabinets

  • Decorative fittings

  • Garden lighting

  • Caravan and motorhome lighting

  • Small pendant lights

Important: Before replacing a G4 lamp, always check whether your fitting requires a 12V low-voltage bulb or a mains-voltage alternative. Never install a bulb with the wrong voltage.