Which Light Bulb Do I Need?

Standing in front of a fitting with a dead bulb in one hand and a dozen lookalikes on screen is exactly how a five-minute job turns into a guessing game. Most bulb problems come down to four things: the cap type, the brightness, the colour temperature, and whether the fitting needs anything specific such as dimmable or LED-only.

Get those right and the rest is straightforward. Get one wrong and the bulb either will not fit, will not look right, or will not work as expected.

Which light bulb do I need? Start with the fitting

The quickest way to answer the question “which light bulb do I need” is to check the old bulb first. If you still have it, look at the metal base or ceramic body. Most bulbs have the fitting printed on them.

In UK homes, the most common cap types are bayonet and Edison screw. A standard bayonet cap is B22. A standard Edison screw is E27. Smaller fittings are also common, especially in lamps, cooker hoods, bathroom mirrors and spotlights. You may see B15, E14, GU10 or G9.

If the bulb is missing, check the light fitting itself. Many shades, pendants and spotlight housings have a label showing the correct bulb type and the maximum wattage. If that label has worn off, the shape of the holder usually gives it away. A push-and-twist fitting is often GU10. A screw-in holder is usually E14 or E27. A bayonet holder takes B22 or B15.

This matters because bulb shapes can look similar while the fitting is completely different. A candle bulb might come with a small screw or a small bayonet cap. A spotlight might be GU10 mains voltage or a different low-voltage fitting altogether. Close is not good enough here.

Common UK bulb fittings explained

B22 is the standard bayonet cap found in many ceiling lights, table lamps and older fittings. It pushes in and twists to lock. If your home has older light fittings, this is one of the first types to check.

E27 is the large Edison screw. It twists into place and is common in pendant lights, larger lamps and many decorative fittings. E14 is the smaller version, often used in bedside lamps, wall lights and chandelier-style fittings.

GU10 is one of the most common spotlight bulbs in UK kitchens, bathrooms and hallways. It has two short pins with a twist-lock action and usually runs on mains voltage, which keeps replacement simple.

G9 capsules are small loop-pin bulbs used in compact fittings and some decorative lights. They are bright for their size but can run hotter in older halogen versions, so LED replacements are often the better option.

If you are unsure, do not rely on bulb shape alone. Check the cap code or compare the base carefully before you add anything to basket.

Brightness matters more than wattage

A lot of people still shop by wattage because that is what older bulbs used. With LED bulbs, wattage tells you how much electricity the bulb uses, not how bright it looks. For brightness, look at lumens.

As a rough guide, an LED bulb around 470 lumens gives a similar light output to an old 40W incandescent bulb. Around 800 lumens is close to a traditional 60W bulb. Around 1,050 to 1,100 lumens is closer to the old 75W range.

This is one of the easiest ways to avoid disappointment. If a room feels gloomy after you change bulbs, the fitting may be correct but the lumen level is too low. For a bedside lamp, that may be fine. For a kitchen or home office, it usually is not.

It also depends on how the room is used. A hallway light can be softer. A bathroom mirror light needs clearer brightness. A utility room often benefits from stronger, cooler light so you can actually see what you are doing.

Choose the right colour temperature

Bulb colour is usually shown in Kelvins, written as K. This affects the feel of the room more than many people expect.

Warm white, usually around 2700K to 3000K, gives a softer yellow-toned light. It suits lounges, bedrooms and table lamps where you want a more comfortable feel.

Cool white, often around 4000K, is cleaner and sharper. It works well in kitchens, bathrooms, garages and workspaces. Daylight bulbs, around 5000K to 6500K, are even cooler and brighter-looking. Some people like them for task lighting, but they can feel harsh in living spaces.

There is no single best option for the whole house. That is where many people go wrong. One bulb that is ideal in a desk lamp can feel far too clinical in a lounge. If you want a safe all-round choice, warm white suits most living areas and cool white suits most practical task areas.

Check if you need dimmable, LED or a special bulb

Not every bulb works in every fitting, even if the cap matches. If the light is on a dimmer switch, you need a dimmable bulb, and in many cases an LED-compatible dimmer as well. If you fit a non-dimmable bulb on a dimmer circuit, you may get flickering, buzzing or a shortened bulb life.

Enclosed fittings are another thing to watch. Some bulbs are not suitable for fully enclosed shades or sealed outdoor fittings because heat builds up. Bathrooms and outdoor lights may also need a bulb suitable for the fitting’s rating and conditions.

Spotlights deserve a quick extra check. GU10 LED replacements are common, but beam angle can vary. A narrow beam gives a more focused spotlight. A wider beam spreads light more evenly. If you are lighting a kitchen worktop, that difference shows straight away.

For decorative lights, filament-style LED bulbs are popular because they give the old-fashioned look without the energy waste of traditional incandescent bulbs. They are practical, but you still need to match fitting, lumen output and bulb size.

Size and shape can catch you out

Even when the cap is correct, the bulb body may be too wide, too long or the wrong shape for the shade. This happens a lot with small lamps, cooker hoods, tight wall lights and decorative fittings.

If the old bulb sat inside a glass cover or narrow shade, compare dimensions before buying. Standard GLS bulbs, candle bulbs, golf ball bulbs and reflector spotlights all take up space differently. A bulb that technically fits the holder may stick out awkwardly or press against the shade.

This is especially worth checking when switching from older halogen to LED. LED versions sometimes have a slightly different body shape or heat sink design.

Which light bulb do I need for each room?

For living rooms and bedrooms, most people are happiest with warm white bulbs. A B22 or E27 fitting is common in main ceiling lights, while lamps often take E14, E27 or B22 depending on age and style.

For kitchens, cool white is often the better practical choice. Ceiling fittings may use B22 or E27, while under-cabinet or spotlight setups often use GU10 or smaller specialist bulbs.

For bathrooms, check the fitting first and keep an eye on suitability for the location. Mirror lights and compact fittings often use G9 or small screw caps. Cool white tends to work well here because it gives a cleaner light.

For hallways, landings and cupboards, lower-lumen bulbs can be enough if the area is small. If the fitting stays on for long periods, LED is the sensible option for lower running costs.

For home offices, garages and utility rooms, choose enough lumens and a cooler light that helps with visibility. A dim, warm bulb might look cosy, but it is not much use when you are reading labels, using tools or sorting laundry.

If you’re replacing an old bulb, do this

Start with the code on the old bulb. Then check the lumen rating, colour temperature and whether it was dimmable. If the old bulb always felt too dim or too yellow, treat the replacement as a chance to improve things rather than copy the same problem.

If the bulb failed unusually quickly, think about the fitting as well. Heat, poor ventilation, incompatible dimmer switches and frequent switching can all affect lifespan. The bulb itself is not always the only issue.

If you are buying for more than one room, it often makes sense to sort the whole lot in one go so you are not making repeat orders for one missed cap type or one overlooked spotlight.

A light bulb should be one of the simplest jobs in the house. Usually it is, once you know what to check. Match the fitting first, buy by lumens rather than old wattage habits, and pick a light colour that suits the room rather than forcing one type everywhere. If you do that, you will usually get it right first time.