How to choose the right bulb fitting for your home

Why getting the fitting right matters

Few household jobs are as satisfying as swapping a blown bulb and watching a room spring back to life. Few are as irritating as discovering the replacement you have just bought will not fit, or will only screw in halfway. The difference is almost always the cap — the base of the bulb that connects to the lampholder. Get it right and you are done in thirty seconds. Get it wrong and you risk a loose connection that can arc, overheat, flicker or fail early. It is worth taking a minute to learn the handful of fittings used in British homes.

Most of our homes use one of two families: bayonet (push and twist) or screw. Alongside those sits a smaller group of pin-based and twist-lock lamps, mostly found in downlights and decorative fittings.

Bayonet fittings: the classic British cap

If you grew up in Britain, the bayonet is probably the cap you picture when you think of a light bulb. It has two small pins sticking out from the side of a cylindrical base; you push it into the holder, then twist about a quarter turn until it seats. It is quick, secure and it will not work loose from vibration.

• BC (B22) — 22mm across the cap. The standard bayonet, used in most ceiling pendants and a great many table and floor lamps.
• SBC (B15) — 15mm across. The small bayonet, common in picture lights, small shades, cooker hoods and some appliance lamps.
• Specialist variants such as three-contact versions exist for dual-wattage or multi-switch lamps, but they are unusual in domestic lighting.

Bayonet caps are generally fitted to mains-voltage lamps, so you rarely need to think about transformers. That simplicity is a big part of why they remain the default in so many British fittings.

Screw fittings: Edison and small Edison

Screw caps, often called Edison screw, simply turn into the holder until they are snug. Do not overtighten — firm is enough. Being able to tighten or loosen them gradually makes them convenient, but the trade-off is that a poorly seated screw bulb can work loose over time, particularly on a pendant or a lamp that gets moved.

• ES (E27) — 27mm across the threaded base. Widely used in table lamps, floor lamps and many ceiling fittings.
• SES (E14) — 14mm across. The small screw, standard in chandeliers and candelabra-style fittings, small wall lights, bedside lamps and many decorative shades.
• MES (E10) — 10mm. The miniature screw, occasionally found in fairy lights, display cabinets and specialist decorative lamps.

Because both bayonet and screw lamps are available in matching shapes, the cap is usually the only thing you need to check. A candle bulb, for instance, comes in both B22 and E14 versions.

Pin, twist-lock and integrated fittings

This is where people most often buy the wrong thing. These fittings look similar but are not interchangeable, and the voltage matters just as much as the shape.

• GU10 — mains-voltage twist-lock with two 10mm pins, standard in recessed downlights and track spots.
• GU5.3 / MR16 — 12V bi-pin, almost always fed from a transformer or driver. It will not work in a GU10 holder, and forcing it can damage both.
• G9 — a mains-voltage capsule with two looped pins, common in small decorative fittings and some bathroom lights.
• G4 — a tiny 12V bi-pin, typically used in cabinet and display lighting.
• R7s — a long linear lamp, 78mm or 118mm, still found in some wall floodlights.
• Integrated LED — no replaceable bulb at all. If the light source is a sealed panel or a built-in strip, you replace the whole fitting when it fails.

How to identify the fitting you already have

Before you buy anything, work through these checks. They take less than a minute and will save you a wasted trip.

• Read the old bulb. The cap code — B22, E14, GU10 and so on — is usually printed on the base or the side of the lamp.
• Measure the cap. 22mm means BC, 15mm means SBC, 27mm means ES, 14mm means SES.
• Check the voltage for pin lamps. GU10 is mains; MR16 and G4 are low voltage and need a working driver.
• Look at the fitting itself. Many lampholders carry a small label stating the fitting type and the maximum wattage.
• Photograph it. A picture of the old bulb next to a ruler is the quickest way to get the right match in a shop.

Also note the shape while you are there — GLS, candle, golf ball, globe or reflector — and roughly how long the bulb is, so it clears the shade.

What to look for when you buy

Once you know your cap, the rest is about matching the light to the room. Buy by lumens rather than watts: roughly 400 lumens replaces an old 40W bulb, 800 lumens replaces 60W, and 1,100 to 1,500 lumens covers a bright living space or kitchen. For colour, choose around 2700K for living rooms and bedrooms, and 4000K for kitchens, bathrooms and utility areas where you want crisper light.

• Dimming: only use dimmable lamps on a dimmer, and check compatibility with your switch — older dimmers often need a trailing-edge replacement to avoid flicker.
• Size: confirm the bulb fits inside the shade's collar, not just the holder.
• Finish: frosted or opal lamps cut glare under a pendant; clear lamps suit decorative filament fittings.
• Location: choose the correct IP rating for bathroom zones and outdoor fittings.
• Quality of light: a colour rendering index of 90 or above makes a noticeable difference in mirrors and over artwork.

Buy two of anything awkward — a spare in the cupboard beats a dark hallway on a Sunday evening.