GU10 vs E27 vs B22: A UK Bulb Fitting Comparison
You've taken the dead bulb out of the fitting and you're holding a cap you can't quite name. The packaging in the shop looks identical, the wattage matches, and the lamp still refuses to go in. That's usually a fitting mismatch rather than a faulty bulb.
Three caps cover the great majority of UK domestic lighting: GU10, E27 and B22. Learn to tell them apart, and to spot the near-misses, and replacing a bulb becomes a two-minute job instead of a guessing game.
What the code on the cap actually means
The letters and numbers describe the physical connector, not the light it produces. E is an Edison screw. B is a bayonet. G is a pin cap. The number tells you the size in millimetres, measured across the screw or across the pins.
So E27 is a 27mm screw cap, B22 is a 22mm bayonet, and GU10 has two pins spaced 10mm apart. Once it's written out like that, the code stops looking cryptic — and it explains why two lamps that seem almost identical on the shelf can be completely incompatible.
You'll also meet the small versions: E14 (small screw, 14mm) in candle bulbs and small table lamps, and B15 (small bayonet) in decorative fittings. Both are widely available in LED form, but neither is interchangeable with its larger relative.
GU10: the spotlight cap
A GU10 pushes in and locks with a quarter turn. Two stubby pins, mains voltage, no transformer or driver in the ceiling to think about — the electronics sit inside the lamp itself. Most are around 50mm across and shaped like a small reflector, which is why they sit neatly in recessed downlights.
You'll find GU10 fittings in kitchen ceilings, bathroom spotlights, track lighting, display cabinets and some wall-mounted reading lights. Because they're directional, they throw a cone of light rather than a general glow, so beam angle matters as much as brightness.
The near-miss to watch for
GU5.3 — sold as MR16 — looks frustratingly similar, but its two pins are thinner and spaced 5.3mm apart. It is usually a 12V low-voltage lamp, which means it often relies on a transformer or driver hidden in the ceiling. A GU10 is mains voltage and has fatter pins set 10mm apart. If you try to force a GU5.3 into a GU10 holder, you risk bending pins and damaging the fitting.
Check the old lamp before you buy. If it says 12V, you need an MR16/GU5.3 replacement. If it says 230V–240V and has two chunky pins, you need GU10. The fitting itself may also carry a voltage label, so look at the holder as well as the dead bulb.
GU10 buying notes
When replacing a GU10, don't simply match the old wattage. Halogen GU10s were often 50W, while modern LED equivalents produce similar light for around 4W–7W. Look at lumens instead.
- Beam angle: narrow beams around 24°–36° suit accent lighting and artwork; wider beams around 60°–100° work better for general kitchen or bathroom light.
- Dimmability: only buy a dimmable GU10 if the circuit is on a dimmer, and check the dimmer is compatible with LED loads.
- Colour temperature: warm white around 2700K feels cosy; cool white around 4000K suits task areas; daylight around 6000K is bright and clinical.
- Physical size: some LED GU10s are longer than the halogen lamps they replace, so check they fit inside the downlight trim.
E27: the standard screw cap
E27 is the large Edison screw, 27mm across. It screws in clockwise and is the classic cap for table lamps, floor lamps, pendant lights and many ceiling fixtures. In UK homes it is especially common in freestanding lamps and shades that take a general-service bulb.
The E27 cap is simple, robust and easy to replace. Because it is a screw fitting, the centre contact and the threaded outer contact must line up properly. Screw it in firmly, but don't over-tighten it — the cap should feel snug, not forced.
E27 near-misses
The biggest confusion is E26. That is the North American standard, 26mm across, and it is close enough to E27 that some people try to make it work. It may screw in loosely or make poor contact, which can cause flickering or overheating. If you're buying from a UK retailer, check the label says E27, not E26.
E14 is the small screw cap, often found in candle bulbs, chandeliers and small decorative lamps. It is not an E27 with a smaller bulb shape; the cap itself is smaller. A B22 bayonet will not screw into an E27 holder at all, and an E27 will not push into a bayonet fitting.
If a bulb screws in but feels loose, or flickers when you touch the lamp, don't ignore it. The wrong cap or a worn holder can be a safety issue, not just an annoyance.
B22: the bayonet cap
B22 is the standard bayonet cap, 22mm across, with two locating pins on the side of the cap. You push it in and twist it clockwise until it locks. In the UK, B22 is still extremely common in older ceiling roses, pendant fittings, wall lights and many household lamps.
The bayonet action is quick and secure, which is why it remains popular. Unlike a screw cap, it gives a clear click or stop when seated. If it doesn't twist and lock, you either have the wrong cap or you haven't aligned the pins with the slots.
B22 near-misses
B15 is the small bayonet cap, 15mm across. It appears in smaller decorative fittings, some chandeliers and compact lamps. It looks like a B22 that has been shrunk, but the pins and cap diameter are different. A B22 will not fit a B15 holder, and a B15 will be too small and loose in a B22 fitting.
There is also the bayonet-versus-screw trap. A B22 and an E27 may produce the same light, but their bases are completely different. Never try to screw a bayonet cap into a screw holder or push a screw cap into a bayonet fitting.
How to identify your fitting in 30 seconds
- Switch off the power at the wall and let the old bulb cool completely.
- Remove the old bulb carefully. If it turns, it's likely a screw cap. If it pushes and twists, it's likely a bayonet. If it has two pins and no screw thread, it's a pin cap.
- Look at the cap. A wide screw is usually E27; a wide bayonet is usually B22; two fat pins are usually GU10.
- Measure if unsure. Across the screw or cap diameter is quickest: about 27mm for E27, 22mm for B22, and 10mm between pins for GU10.
- Check the label or old bulb. The cap code is often printed on the bulb itself, the fitting, or the packaging of the old lamp.
Matching the light, not just the fitting
Once you know whether you need GU10, E27 or B22, the next job is choosing the right light output. A bulb that fits but feels wrong will still make the room feel wrong.
- Brightness: compare lumens, not watts. A 60W traditional bulb was roughly 700–800 lumens; a 100W bulb was roughly 1500–1600 lumens. LED equivalents use far fewer watts.
- Colour temperature: 2700K–3000K for warm living spaces, 4000K for kitchens and bathrooms, and 6000K for garages or task lighting.
- Dimmability: if you have a dimmer switch, choose dimmable lamps and check compatibility. Non-dimmable LEDs may buzz, flicker or fail early.
- Shape and size: candle, globe, golf ball, reflector and classic shapes all come in different caps. A GU10 spotlight is not the same as a GU10-shaped decorative lamp.
Quick comparison
- GU10: two 10mm pins, push and twist, mains voltage, usually directional spotlights for downlights and track lighting.
- E27: 27mm Edison screw, screws in, mains voltage, common in table lamps, floor lamps and pendants.
- B22: 22mm bayonet, push and twist, mains voltage, common in older UK ceiling fittings, wall lights and lamps.
Common mistakes and simple fixes
- Forcing the bulb: if it doesn't turn or click easily, stop. Check the cap code before applying more pressure.
- Ignoring voltage: GU10 is normally mains voltage; GU5.3/MR16 is often 12V. Mixing them up can damage the lamp or fitting.
- Buying by wattage alone: LED wattage is much lower than halogen or incandescent wattage. Use lumens as your guide.
- Using a non-dimmable LED on a dimmer: it may flicker, buzz or fail. Choose dimmable lamps and a compatible dimmer.
- Assuming all small caps are the same: E14 is not E27, and B15 is not B22, even if the bulbs look similar in a photo.
Final word
GU10, E27 and B22 are not competing technologies; they are three different ways of connecting a bulb to a fitting. GU10 is the push-and-twist spotlight cap, E27 is the standard screw cap, and B22 is the standard bayonet cap. Once you can recognise the cap, the voltage and the light output you need, the shop shelf stops being a puzzle. Take the old bulb with you if you're unsure — it is still the fastest way to get the right replacement first time.
Photo: La Miko / Pexels
