Ordering the wrong bulb cap type is the most preventable lighting mistake in the UK. GU10 will not screw into an E27 holder. B22 will not twist into a GU10 socket. The fittings are physically incompatible and there is no workaround beyond a cap adaptor, which itself requires checking compatibility. The correct approach: identify your fitting type before ordering. This guide covers every common UK bulb cap type and how to identify each one without specialist knowledge.
Key Takeaways
- GU10: twist-lock, two small pins, used in downlights and spotlights
- E27: large Edison screw (27mm diameter), used in pendants and floor lamps
- E14: small Edison screw (14mm), used in chandeliers, candelabra fittings, and small decorative lamps
- B22: bayonet cap with two side pins, the traditional UK standard in pendants and table lamps
- G9: two small looped pins, used in wall lights and some smaller ceiling fittings
- If in doubt, remove an existing bulb from the fitting and read the cap designation printed on the base
The quick answer: GU10 is the standard for recessed downlights and surface spotlights in UK homes built or renovated in the past 20 years. E27 is the standard for decorative pendants, floor lamps, and most table lamps. B22 bayonet is the older UK standard still found in many existing pendants and table lamps. E14 is used in chandelier arms and small decorative fittings. The cap designation is always printed on the base of the bulb. Remove one existing bulb and read the marking if you are unsure which type your fitting takes.
How to Identify Your Bulb Cap Type
The cap designation is printed or embossed on the metal base of every bulb and on every lamp holder. GU10 appears as "GU10" on both the bulb cap and inside the fitting holder. E27 appears as "E27" or "ES" (Edison Screw). B22 appears as "B22" or "BC" (Bayonet Cap). E14 appears as "E14" or "SES" (Small Edison Screw).
If the fitting is empty and you cannot find the label, the physical shape of the holder identifies the type. GU10 holders have a circular socket with two rectangular slots at roughly 2 o'clock and 8 o'clock positions. E27 and E14 holders have a threaded cylindrical interior. B22 holders have two small square channels on the inside of the cylinder where the bayonet pins engage.
GU10 Bulbs: The Standard for UK Downlights and Spotlights
GU10 is the dominant cap type for recessed ceiling downlights and surface-mounted spotlights in UK homes built since approximately 2005. The "GU" designation means "Glass holder, U-shaped pins." The "10" is the pin spacing in millimetres. The cap has two small pins at the base of the bulb. Insert the bulb, rotate a quarter-turn clockwise, and the pins lock into the holder.
GU10 LED replacement bulbs are available in output from around 300 lumens (for accent use) to 1,100 lumens (for high-output task lighting). Standard residential GU10 replacements for a 50W halogen are 5-8W, delivering 600-800 lumens. The bulb diameter is typically 50mm.
One important note: GU10 fittings in the UK ceiling are almost always recessed in a housing. That housing may or may not be fire-rated and insulation-compatible. If your GU10 downlights are in a ceiling adjacent to roof insulation, check whether the housing is rated IC (insulation contact) before purchasing high-output bulbs. Non-IC rated downlights in contact with insulation are a fire risk.
E27 and E14 Fittings: Edison Screw Caps for Pendants and Decorative Lights
E27 (large Edison screw) is the most common decorative bulb fitting in Europe and the UK. The "E" designation means Edison. The "27" is the thread diameter in millimetres. E27 is used in pendant lights, floor lamps, table lamps, and large decorative ceiling fittings. Most decorative LED bulbs (globe, ST64, carbon-filament-style) are available in E27.
E14 (small Edison screw) is 14mm thread diameter. It is used in chandelier arms, candelabra fittings, wall lights with small sockets, and smaller decorative pendants. E14 candle bulbs are the standard replacement for chandelier-style multi-arm fittings. E14 G9 capsule fittings are a separate type (see below).
E27 and E14 are interchangeable in terms of screw thread design but not interchangeable in physical size. An E14 bulb will physically fit into an E27 holder with an adaptor ring, but the reverse (E27 into E14) will not work and should not be attempted. Adaptors are available but check that the maximum wattage of the fitting is not exceeded when using an E27 bulb in an adapted E14 holder.
B22 and Other UK Bulb Fittings
B22 (bayonet cap, 22mm) is the original UK residential lighting standard. The bayonet mechanism predates the Edison screw and is more common in older UK homes. You push the bulb in against spring tension, rotate a quarter-turn, and the pins engage in the slots. B22 is still found in many pendant ceiling fittings, table lamps, and floor lamps in homes built before 2000 and in many fittings since.
G9 (push-in capsule, looped pins) is used in wall lights, some smaller ceiling fittings, and undercabinet lights where the fitting is too compact for a screw cap. The two looped pins push directly into matching holes in the holder. G9 LED replacements are available for most common halogen G9 capsule sizes.
G4 (small push-in, 4mm pin spacing) is used in very small decorative fittings, some undercabinet strip lights, and older landscape garden lights. G4 LEDs are available but are less common than G9 in current residential use.
| Cap Type | Connection Method | Typical UK Application | Common LED Replacement Wattage |
|---|---|---|---|
| GU10 | Twist-lock (quarter turn) | Recessed downlights, spotlights | 5-8W (replaces 50W halogen) |
| E27 | Screw (clockwise) | Pendants, floor lamps, decorative | 6-15W (replaces 40-100W incandescent) |
| E14 | Screw (clockwise, smaller) | Chandelier arms, candelabra, small decorative | 4-6W (replaces 25-40W incandescent) |
| B22 | Bayonet push-and-twist | Traditional pendants, table lamps | 6-15W (replaces 40-100W incandescent) |
| G9 | Push-in (looped pins) | Wall lights, compact ceiling, undercabinet | 3-6W (replaces 25-40W halogen) |
| G4 | Push-in (straight pins, 4mm) | Very small decorative, some garden | 1-2W (replaces 10-20W halogen) |
For recessed GU10 fittings, see recessed spotlights and surface mounted spotlights. For E27 and B22 pendants, browse the pendant lights collection. Lighting accessories covers lamp holders, cap adaptors, and replacement components for all common fitting types.
Frequently Asked Questions About Bulb Fitting Types
How do I know if I have a GU10 or E27 fitting?
Remove an existing bulb. GU10 has two small pins at the base that lock by twisting. E27 has a threaded screw base, 27mm diameter. If the fitting is empty, look inside the holder: GU10 holders have two rectangular slots; E27 holders have a threaded cylindrical interior.
Can I use an E27 bulb in a B22 fitting?
Not without an adaptor. B22-to-E27 adaptors are available and are a practical solution when you want to use a decorative E27 LED (globe, filament style) in a B22 ceiling fitting. Check that the adaptor does not exceed the fitting's maximum wattage and that the combined bulb-plus-adaptor weight is within the fitting's mechanical limit.
What is the difference between E27 and E14?
Thread diameter. E27 is 27mm, used in larger decorative pendants, floor lamps, and table lamps. E14 is 14mm, used in chandelier arms, candelabra fittings, and smaller decorative lamps. They are physically different sizes and are not interchangeable without an adaptor.
How do I remove a GU10 bulb that is stuck?
Turn off and wait for the bulb to cool. GU10 bulbs can stick in the holder after heat cycling. Push gently inward while rotating anti-clockwise. If the bulb is very stuck, use rubber gloves for grip. Avoid pulling without rotating, as this can break the ceramic holder. If the pins remain in the holder after the glass breaks, switch off at the fuse box and use pliers to rotate the remaining cap.
Are GU10 and MR16 bulbs the same?
No. GU10 is mains voltage (230V) with a twist-lock fitting. MR16 is low voltage (12V) with a push-pin fitting. They are not physically interchangeable. GU10 fittings are far more common in UK homes. MR16 fittings require a separate 12V driver on the circuit. If you are unsure which type you have, check the existing bulb: GU10 bulbs have a smooth dome and a wide base; MR16 bulbs have two thin straight pins, 50.8mm apart.
