Buying a replacement bulb should be simple, but the codes on the box can make it feel more complicated than it is. The most important thing to get right is the fitting: the physical connection between the bulb and the holder. In UK homes, some of the most common light bulb fittings are E14, E27, GU10 and B22. They look different, connect differently and are not interchangeable.
Once you know which fitting you already have, choosing a replacement becomes much easier. This guide shows you how to identify the fitting, what each code means, and what else to check before you buy.
Quick comparison of UK light bulb fittings
The quickest way to narrow down the different light bulb fitting types is to look at how the bulb connects to the holder.
| Fitting | Connection | Key measurement | Common UK use | Interchangeable? |
|---|---|---|---|---|
| E14 | Small Edison screw | 14mm base diameter | Smaller lamps, candle style fittings, decorative fixtures | No. Requires an E14 holder |
| E27 | Edison screw | 27mm base diameter | Table lamps, pendants and general household fixtures | No. Requires an E27 holder |
| GU10 | Two pin twist and lock | 10mm pin spacing | Spotlights and many recessed downlights | No. Requires a GU10 holder |
| B22 | Bayonet cap with side pins | 22mm cap diameter | Traditional UK lamps and ceiling fittings | No. Requires a B22 holder |
These measurements follow the naming conventions used in UK fitting guides from Toolstation.
How to tell which bulb fitting you have
If you are asking “what bulb fitting do I have?”, start with the bulb that is already installed rather than guessing from the room or fixture style.
Check the code on the old bulb or fitting
Switch the light off and allow the bulb to cool before handling it. Electrical Safety First recommends switching off before changing a lamp. If the old bulb is safe to remove, check the base, body or printed label for a code such as E14, E27, GU10 or B22. Taking a quick photo before shopping can save a lot of guesswork.
Identify the connection shape
E14 and E27 have a threaded metal base that screws into the holder. B22 has two small side pins and is pushed in before being twisted to lock. GU10 has two short pins with enlarged ends; the bulb is inserted and rotated into place.
Measure only when the code is missing
If no code is visible, measurement can help confirm what you are looking at. E14 and E27 refer to screw bases of roughly 14mm and 27mm respectively, while B22 refers to a bayonet cap around 22mm wide. GU10 uses pins spaced 10mm apart. Treat the measurement as a confirmation step, not a reason to force a bulb into a holder that looks different.
The four fittings explained
E27 bulbs: the larger Edison screw
E27 is one of the easiest fittings to recognise. It has a relatively wide threaded base and screws into the holder. The “E” refers to the Edison screw style, while the number identifies the approximate diameter of the base in millimetres.
E27 is common in general purpose lamps and fixtures, but the fitting alone does not tell you the bulb’s shape, brightness or features. An E27 smart bulb and a basic E27 LED bulb can fit the same type of holder while behaving very differently.
For a smart option, Zyko Smart RGB+CCT E27 Bulb is currently listed with an E27 base, 10W output and 800 lumen brightness, with RGB+CCT light from warm to cool white.
E14 bulbs: the smaller Edison screw
E14 uses the same screw in principle as E27, but the base is much narrower. In the UK it is often described as a Small Edison Screw, or SES. E14 fittings are common in smaller lamps and decorative fixtures, including many candle style lamps.
That does not mean every E14 bulb is candle shaped. E14 describes the base, not the glass or plastic body above it. This distinction matters when comparing E14 vs E27: the first question is which screw holder your fixture has, and only then whether the bulb body will physically fit inside the shade or enclosure.
Zyko Smart RGB+CCT E14 Candle Bulb is currently listed with an E14 base, 5W output and 550 lumens, plus RGB+CCT warm to cool white light.
GU10 bulbs: the twist and lock spotlight fitting
GU10 looks completely different from an Edison screw. Instead of a threaded base, it has two short pins with wider ends. You push the bulb into the holder and twist it to lock it in place. GU10 is widely associated with spotlights and recessed downlights.
If you are comparing E27 vs GU10, there is no direct swap: one screws in and the other locks into a two pin holder. GU10 should also not be confused with GU5.3/MR16 style fittings. Those use a different base, so check the existing holder rather than relying on the bulb’s overall spotlight shape.
Zyko Smart RGB+CCT WiFi GU10 Spotlight is currently listed with a GU10 base, 5W output and 500 lumens, with RGB+CCT warm to cool white light.
B22 bulbs: the UK bayonet cap
B22 is a familiar fitting in many UK homes. Instead of screwing into a thread, the cap has two side pins. You line the pins up with the holder, push the bulb in and twist it to secure it.
When comparing B22 vs E27, the key difference is the connection itself. B22 is a bayonet fitting; E27 is an Edison screw. A bulb with one base will not directly fit a holder designed for the other. Zyko does not currently list a B22 bulb, but it belongs in this guide because it remains a common fitting that UK shoppers are likely to encounter.
E14 vs E27 vs GU10 vs B22: which one do you need?
None of these fittings is universally “better”. Your existing holder decides which fitting you need.
- Choose E14 when your fixture has the smaller Edison screw holder.
- Choose E27 when it has the larger Edison screw holder.
- Choose GU10 when it uses the two pin twist and lock spotlight connection.
- Choose B22 when it uses the push and twist bayonet cap with side pins.
This is why E14 vs E27, E27 vs GU10 or B22 vs E27 should be treated as compatibility questions, not performance contests. The fitting tells you whether the bulb can connect. It does not tell you which bulb is brighter, smarter or more suitable for your room.
What else to check after the fitting
Finding the correct base is step one. Before ordering, check the rest of the bulb specification as well.
Check physical dimensions and shade clearance
Two bulbs can share the same fitting while having different body shapes or overall sizes. A bulb that connects correctly may still be too wide or tall for a compact shade or decorative fitting. Compare the replacement bulb’s dimensions with the space available.
Compare lumens, not just watts
Watts describe power use, while lumens describe the amount of light produced. The Energy Saving Trust recommends using lumens when comparing brightness, which is especially useful when replacing older bulbs with LEDs.
Check colour temperature and controls separately
The correct fitting does not automatically guarantee the colour, white tone or control features you want. If you are choosing between warmer and cooler white light, our guide to how colour temperature affects mood and productivity explains that side of the decision in more detail.
For smart bulbs, also check the product’s stated control requirements and compatibility before buying. Do not assume that two bulbs with the same fitting support the same app, dimmer, smart home platform or installation environment.
Before you buy: a six point fitting checklist
- Confirm the fitting code on the old bulb or holder where possible.
- Match the connection type: screw, bayonet or twist and lock.
- Check the bulb’s physical size against the shade or fixture.
- Compare lumens so the replacement gives the level of brightness you expect.
- Choose the colour or white tone that suits the space.
- Verify any extra compatibility you need, such as smart controls or dimming, from the product specification.
If anything about the holder looks damaged, loose or unusual, avoid improvising with adapters or electrical modifications. Changing a normal bulb is one thing; faults or wiring work should be handled appropriately.
Frequently asked questions
Is GU10 the same as E27?
No. GU10 uses a two pin twist and lock connection, while E27 uses a 27mm Edison screw. They require different holders.
Which is better, E14 or E27?
Neither is inherently better. E14 is the smaller 14mm Edison screw and E27 is the larger 27mm screw. The holder in your fixture determines which one fits. After that, compare brightness, bulb dimensions and features.
Can I put an E14 bulb in an E27 socket?
Not directly. The screw bases are different sizes, so an E14 bulb will not fit an E27 holder by itself. It is safer to buy the correct fitting rather than assume an adapter is suitable for a particular fixture.
How do I know what light bulb fitting I have?
Switch the light off, let the bulb cool, and check the old bulb or holder for a printed fitting code. Then compare the connection shape: E14/E27 screw in, B22 uses side pins, and GU10 uses two twist lock pins. Measure only if the code is missing.
Is B22 the same as E27?
No. B22 is a bayonet cap that pushes and twists into place, while E27 is a screw fitting. The two bases are physically different and are not directly interchangeable.
Match the fitting first
The simplest way to avoid buying the wrong bulb is to solve compatibility before features. Identify the base you already have, match it exactly, then compare size, lumens, colour temperature and any smart controls you actually need.
For UK homes, recognising the difference between E14, E27, GU10 and B22 covers many of the most common light bulb fittings. Once that part is clear, choosing the right replacement becomes a much more straightforward job.
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