You've found the perfect light fitting. You take it home, unbox it, walk over to the cupboard where you keep spare bulbs — and none of them fit. The fitting wants something called a GU10. You've got E27s, B22s, even some E14s, but no GU10. So back to the shop.
It's one of the small frustrations of modern home lighting: there are four different bulb cap types in common use in the UK, they're not interchangeable, and the labels look like an industrial standards document.
Here's the plain explanation. What each cap type is, where you'll find it, and how to make sure you never have to make a second trip.
The four cap types in UK homes
Modern British lighting uses four cap types as standard. Almost any new light fitting you buy will take one of these. The names look intimidating but the logic is straightforward once you know how they're constructed.
- GU10 — twist-and-lock with two short pins. Mains voltage. Used in spotlights, downlights, and many modern pendants.
- E27 — Edison screw, large diameter (27mm). The "classic lightbulb" cap. Used in most table lamps, floor lamps, and decorative pendants.
- E14 — Edison screw, small diameter (14mm). Used in chandeliers, smaller decorative fittings, and many wall sconces.
- B22 — bayonet cap. Two pins on the sides, push-and-twist to lock. The older British standard, still common in legacy fittings.
The naming follows international convention: the "E" stands for Edison, the inventor of the screw cap, with the number indicating the diameter in millimetres. The "GU" prefix is the formal designation from the International Electrotechnical Commission (IEC), the global body that defines bulb cap standards.
GU10 — the modern downlight standard
The GU10 is what you'll find in most spotlights, downlights, and increasingly in modern pendant designs. It's a small, twist-and-lock fitting with two short prongs that protrude from the back of the bulb. You insert it, give it a quarter-turn, and it locks into place.
GU10s run on standard UK mains voltage (240V), which is what makes them simpler than the older MR16 spotlights that needed a separate transformer. If a fitting takes a GU10, you wire it like any normal mains fitting — no extra hardware required.
You'll find GU10 fittings in:
- Recessed ceiling downlights and spotlights
- Track lighting
- Many modern pendant designs (especially Scandinavian / minimalist styles)
- Adjustable wall spotlights
- Cabinet lighting
Available bulb options for GU10 range from very narrow beam angles (12–25°, suitable for accent spotlighting on artwork or features) to wide-angle floods (60° or more, suitable for general ambient lighting). Read the beam-angle spec carefully — a 12° GU10 in a kitchen ceiling will create harsh pools of light surrounded by darkness, while a 60° GU10 will spread evenly. Most domestic applications want 36–60°.
E27 — the workhorse
The E27 is the standard large Edison screw. It's the cap on most "regular" looking bulbs — the classic A-shape light bulb you grew up with, plus most decorative filament bulbs, globe bulbs, and statement bulbs.
If you have a fitting with a visible bulb (a bare-bulb pendant, an exposed Edison-style fixture, a table lamp with a shade) it's almost certainly E27. Most of our pendant lights and table lamps use E27 because it gives you maximum flexibility on bulb choice.
The E27 is also the most flexible cap when it comes to bulb selection. You can choose:
- Standard A60 LED bulbs (the classic light-bulb shape)
- Globe bulbs (decorative, with the bulb itself visible — G80, G95, G125 depending on diameter)
- Decorative filament bulbs (vintage-style, with visible LED filaments)
- High-output bulbs for fittings that need extra brightness
- Smart bulbs (Wi-Fi or Zigbee controlled)
The breadth of choice is why E27 has become the default for any fitting where the bulb itself is part of the look.
E14 — the smaller Edison
The E14 is the same Edison-screw mechanism as E27, just at a smaller diameter (14mm). It's used where the fitting is too compact for an E27 — chandeliers with multiple small bulbs, smaller decorative pendants, fancy bedside lamps, many wall sconces.
Our wall sconces often take E14, particularly the more traditional or classical designs. Chandeliers almost universally use E14 because they need multiple bulbs in close proximity.
Bulb options for E14 are narrower than E27 — you'll find candle-shaped bulbs (the long thin teardrop shape), small golf-ball bulbs, and small filament bulbs. Most are decorative rather than high-output.
B22 — the bayonet cap
The B22 is the older British standard. Instead of a screw thread, it has two pins on the sides of the cap; you push the bulb in and give it a quarter-turn to lock.
B22 was the default in British homes for most of the 20th century, and you'll still find it in many older fittings. It's gradually being phased out in new lighting designs in favour of E27, but the installed base is enormous — particularly in houses built before 1990.
Modern LED bulbs are widely available in B22, so if you've inherited a house with B22 fittings, you don't need to replace the fittings — just buy B22 LED bulbs. The bulb selection is narrower than E27, but covers the standard A-shape and most common decorative options.
The other ones — what you'll occasionally see
You may encounter these less common caps in specific situations:
- MR16 (GU5.3) — low-voltage (12V) spotlight cap, similar to GU10 in shape but requires a transformer. Common in older recessed lighting; mostly being replaced by GU10 in new installs.
- G9 — small two-pin capsule bulb. Used in compact decorative fittings, particularly chandeliers and small ceiling lights where space is at a premium.
- G4 — tiny pin-base capsule, used in some under-cabinet and feature lighting.
- R7s — double-ended linear bulb, used in some halogen floodlights and uplighters.
For mainstream UK domestic lighting, you can mostly ignore these — they appear in specialist contexts but not in everyday decorative pieces.
How to check what your fitting needs
The easiest way: take out the dead bulb and look at the cap. The product page or box of the fitting (if you still have it) will also list the cap type. As a backup, the visual differences are obvious once you know what to look for:
- Two short prongs sticking straight out the back, with a twist-lock motion → GU10
- Continuous spiral screw thread, large (about the width of a 2p coin) → E27
- Continuous spiral screw thread, small (about the width of a 5p coin) → E14
- Two pins on the sides, push-and-twist → B22
Before ordering any new light fitting, check the cap type in the product description and decide whether you already have spare bulbs for it. New fitting + new bulbs = double cost, double waste of time, and a sudden cupboard full of mismatched spares.
Can I convert one cap type to another?
Adapters exist for some cap combinations — B22-to-E27, E27-to-E14, and a few others. They're cheap, widely available, and useful in specific scenarios. But there are real caveats:
Stability — an adapter adds 1–2cm to the bulb's overall height, which may mean the bulb no longer fits inside the intended fitting (a glass shade, a recessed can). Always check clearances before buying.
Heat dissipation — adapters can interfere with how the bulb sheds heat, particularly in enclosed fittings. The bulb may run hotter and have a shorter lifespan.
Safety — cheap adapters from unverified sources have been known to cause electrical issues. Electrical Safety First recommends buying adapters from reputable suppliers with proper CE/UKCA marking, and avoiding stacked adapters (multiple adapters chained together) entirely.
Adapters are a workable short-term fix. If you find yourself relying on them long-term, it's usually a sign that the original fitting choice was wrong, and you'll be better served by a fitting that takes the right cap natively.
Voltage — a quick safety note
Most UK domestic bulbs run at 230–240V mains voltage. GU10, E27, E14, and B22 are all mains-voltage caps. You can safely wire them through standard household electrical circuits.
The exception is MR16 / GU5.3 — these are 12V low-voltage and require a transformer between the mains and the bulb. If you have older recessed lighting that wasn't working properly, this is a common diagnosis — the transformer has failed, not the bulb. Most homeowners upgrading from MR16 to GU10 also remove the transformer (or have an electrician do it) to simplify the circuit.
For any wiring beyond a simple bulb swap, the standard advice from NICEIC and other UK electrical bodies applies: use a Part P-registered electrician for new circuits, alterations in special locations (bathrooms, outdoors), or anything where you're unsure.
Choosing fittings with bulb practicality in mind
When you're picking a new light fitting, the cap type should be on your mental checklist alongside finish, style, and price. A few practical principles:
- Prefer E27 where possible — widest bulb choice, best long-term flexibility, easiest replacement.
- GU10 is fine for spotlights and downlights — it's the right tool for the job. Just be aware you'll need a separate cupboard of GU10 bulbs.
- E14 is fine for genuinely small fittings — but if you've got a choice between an E14 and E27 fitting of similar style, E27 is usually the more practical pick.
- Avoid mixing cap types across the same room — a kitchen with E27 pendants and GU10 downlights and E14 wall sconces means buying three different bulb types every time you restock.
- Check before you buy — every product page should specify the cap type. If you can't find it, ask before ordering.
The Enso take
The cap on a bulb is the most boring part of lighting and the one that most often goes wrong. The fix is preparation, not memorisation. When you're choosing a fitting, take 30 seconds to check what cap it takes. Buy a couple of spare bulbs when you order the fitting. Future-you will be grateful.
Looking through our complete lighting collection, every product page lists the cap type clearly. If you ever can't find it, write to us and we'll dig out the spec. We'd much rather you ask than guess wrong.
And for the deeper picture — what bulb to put in any cap once you've worked out the cap — see our companion guides on bulb colour temperature and lumens vs watts.
