Choosing the right replacement light bulb in the UK can feel overwhelming with dozens of code acronyms like B22, E27, GU10, E14, and MR16. Using the wrong cap size or voltage can result in ill-fitting lamps or short-circuited fittings.
1. Bayonet Cap Light Bulbs (B22 & B15)
Bayonet caps are the traditional standard across United Kingdom households. They feature a push-and-twist mechanism with two side pins on the base.
- B22 / BC (22mm Bayonet Cap): The standard 22mm diameter push-and-twist bulb used in majority of UK ceiling pendants and lamps.
- B15 / SBC (15mm Small Bayonet Cap): A miniature 15mm version commonly found on wall sconces, chandeliers, and bedside touch lamps.
2. Edison Screw Light Bulbs (E27 & E14)
Edison screw bases feature a threaded metal mount that screws clockwise into the socket holder. Originally popular across continental Europe, they are now widespread across modern UK light fittings.
- E27 / ES (27mm Edison Screw): Standard 27mm wide screw base used in contemporary pendant lights, outdoor lanterns, and floor lamps.
- E14 / SES (14mm Small Edison Screw): Miniature 14mm screw base used in candle bulbs, golfball lamps, and appliances.
3. Spotlight Caps (GU10 & MR16 / GU5.3)
Spotlights and downlights require specialized pin bases designed for directional lighting in kitchens, bathrooms, and commercial displays.
- GU10 (240V Mains Twist-Lock): Features two solid metal studs 10mm apart. Operates on standard 240V mains power without a transformer. Twist 90 degrees to lock into place.
- MR16 / GU5.3 (12V Low Voltage 2-Pin): Features two thin needle pins spaced 5.3mm apart. Requires a 12V lighting transformer or LED driver to operate.
4. Linear Fluorescent & LED Tube Sizes (T4, T5, T8 & T9)
Tube lighting is measured by eighths of an inch in diameter ("T" number):
- T4 (12mm diameter): Ultra-compact 2-pin tubes widely used for under-cabinet kitchen counter strip lighting (e.g. 6W, 16W, 20W daylight and warm white).
- T5 (16mm diameter): Slim high-efficiency tubes used in modern office troffers and zapper units.
- T8 (26mm diameter): Classic standard commercial garage and warehouse overhead lighting.
- T9 Circular (29mm diameter): Ring-shaped circular fluorescent tubes used in bathroom ceiling domes and magnifying desk lamps.
Summary: Quick Bulb Cap Comparison Chart
| Fitting Code | Common Name | Base Diameter | Mechanism | Standard UK Voltage |
|---|---|---|---|---|
| B22 / BC | Standard Bayonet Cap | 22mm | Push & Twist | 240V Mains |
| E27 / ES | Standard Edison Screw | 27mm | Threaded Screw | 240V Mains |
| GU10 | Mains Spotlight | 10mm stud spacing | Twist-Lock Studs | 240V Mains |
| MR16 / GU5.3 | Low Voltage Spotlight | 5.3mm pin spacing | Push-fit Needle Pins | 12V AC/DC (Needs Driver) |