The Ultimate Light Bulb Buying Guide

If you’re wondering, “What light bulbs do I need?” you’re in the right place! With so many light bulb options, selecting the right one can be tricky if you don’t know what you need. You have base and fitting to think about, as well as shape, colour temperature, brightness and whether you want it to be dimmable or not.

Luckily, we’ve put together this ultimate light bulb buying guide to direct you through the bulb names, fittings, and all the considerations you need to make to ensure you get the right bulb for your light fitting and room.

Table of contents:

• Light bulb caps and fittings: Which light bulb should I buy?
• How do you identify the type of bulb?
• A quick guide to bulb base codes
• Light bulb brightness
• Light bulb shapes
• Light bulb technology
• What colour temperature bulb do you need?
• Dimmable or non-dimmable?

Light bulb caps and fittings: Which light bulb should I buy?

Light bulb bases, also known as caps, are the part that connects the bulb to the light fitting, and they come in several shapes and sizes that can often be confusing. The bulb type you need to choose will depend on the fitting type you have in your home or commercial space, and buying a bulb with an incompatible cap or base for your fittings will render the bulb useless.

What are the different light bulb types?

Choosing the right light bulb cap fitting is the first essential step to light bulb buying. The most common bulb types in the UK are as follows:

• Edison Screw Cap or E27
• Small Edison Screw Cap or E14
• Bayonet Cap or B22
• GU10 Cap
• G9 Cap

Here is the lowdown on each to help you identify them.

Screw Caps (Edison Bulbs)

The technical details:

Name: Edison Screw bulb

Code: E27 or (ES)

Diameter: 27mm

Name: Small Edison Screw bulb

Code: E14 or (SES)

Diameter: 14mm

The Edison screw cap bulb is named after the inventor of the light bulb, Thomas Edison himself, and is one of the most common types of light bulb caps on the market. Largely unchanged since its invention, the screw cap bulb has a threaded metal base that screws tightly and securely into place within the light fitting.

The most commonly used screw cap bulb is the E27, sometimes shortened to ES, which fits most standard light fittings in the UK. There are also smaller versions (E14 and miniature E10s), which have a narrower screw diameter and are most commonly used for decorative lighting, like candle-shaped bulbs for chandeliers.

Bayonet Caps (Twist and Lock)

The technical details:

Name: Bayonet bulb

Code: B22 or (BS)

Diameter: 22mm

If your light fittings do not take screw cap bulbs, odds are they require bayonet cap bulbs, another common bulb used in the UK. Also known as the ‘twist and lock’ bulb, a bayonet cap bulb has two pins that retract and expand into slots inside the fitting when twisted. A standard bayonet light bulb is 22mm in diameter (B22), but smaller B15 or SBS sizes are available.

GU10 Bulbs

The technical details:

Name: GU10 bulbs

Code: GU10

Diameter between pins: 10mm

Although GU10 caps look very different from traditional bayonet cap bulbs, their ‘twist and lock’ mechanism means they are technically part of the bayonet cap family. The letter ‘G’ indicates that the bulb has a two-pin (or a bi-pin) base, and the number 10 indicates that the pins are 10mm apart.

GU10 lamps were traditionally the base cap for halogen bulbs. However, manufacturers are now making LED retrofit versions, which are much more energy-efficient.

These LED GU10 bulbs are commonly used for kitchen and bathroom spotlights, downlights, domestic recessed lighting, and retail showroom lighting.

Another common GU base is the GU5.3 (also known as the MR16), which is used for similar purposes but uses a simple push-in mechanism instead of a twist and lock. GU5.3 bases were also designed for halogen bulbs before LED retrofits were created.

G9 bulbs

The technical details:

Name: G9 bulbs

Code: G9

Diameter between pins: 9mm

G9 bulbs are compact, bi-pin light bulbs commonly used in household and commercial lighting. They feature a distinctive looped pin base measuring 9mm apart, allowing quick release and installation.

Historically, halogen G9 bulbs emerged in the mid-20th century as a more energy-efficient alternative to traditional incandescent bulbs. However, most G9 bulbs are now LED, offering bright, focused, energy-efficient light, particularly for decorative lighting fixtures, spotlights, under-cabinet lighting and chandeliers.

Less common bulbs

Less common bulb types include R7's, MR16, MR11 and G4, traditionally halogen bulbs. Halogen bulbs have been phased out since September 2018. Still, if you have these light-fitting types, you should easily be able to find retrofit LED versions to replace old ones.

How do you identify the type of bulb?

Nothing is more frustrating than buying a new bulb to get it home and realise it has the wrong type of cap for your fittings. Luckily, each bulb type has a code, which is always displayed on the packaging and bulb base to help you match up your old bulb with the correct new one. Some newer fittings will also have bulb type requirements so you can identify what bulb is needed in your light fittings, even if you don’t have an old bulb.

If you have an old bulb for reference:

The fastest and easiest way to find out which type of light bulb fittings you have (and, therefore, which kind of bulb you need to buy) is to simply remove the bulb that is currently in the fitting and check what type of bulb it is. Most bulbs will have the base code and wattage/lumen levels written on them.

If you don’t have an old bulb for reference:

If there is currently no bulb in your fitting, you will need to look closely at the type of fitting and do a quick diameter measurement.

A quick inspection will also tell you if you require a screw bulb vs a pin bulb. For a screw bulb, the inside of the fitting will have spiralling ridge threads, and for a bayonet cap, the inside of the fitting will be smooth, with two L-shaped slots on either side.

For empty spotlight fittings, you can measure the distance between the pin slots to determine whether you need a GU10, GU5.3 or another sized bi-pin bulb.

A quick guide to bulb base codes

But what do the codes mean? Here is our quick guide to decoding bulb base codes to help you find the right bulb for your light fitting.

The first letter(s) indicate the type of base, for example:

• E= Edison (screw)
• B = Bayonet
• G = Multi-pin / Bi-pin

The number(s) indicates the diameter of the base or distance between the pins in millimetres, for example:

• E27 = Edison 27mm
• B22 = Bayonet 22mm
• E14 = Edison 14mm
• GU10 = Bi-pin 10mm between pins

There is also sometimes a final letter at the end, which simply indicates the number of pins present, for example:

• S = single pin
• D = double pin / bi-pin
• T = triple pin

So, for example, a standard bayonet cap may have a code like this: B22d.

Light bulb brightness

If this is the first time in a long while that you’re shopping for a new light bulb, you may notice that things are a little different now. Most of us have spent years getting used to shopping for light bulbs by wattage, but technology has moved on. With the phase-out of incandescent and halogen bulbs, watts are no longer a relevant measurement of light bulb brightness.

Light bulb brightness is now measured in Lumens, which is purely a measurement for brightness rather than energy consumption. You will find that the wattage indicator on LED light bulbs looks very low, but this is because LED lights require less energy to produce the same amount of light.

Not sure which light bulb will match the brightness of your traditional 40 or 100-watt bulbs? Use our handy Lumens to Watts conversion chart to find the right bulb to suit your light fitting and lighting requirements. Or, if you already have an LED bulb, check the bulb to find the lumen rating.

For more information on what brightness level you need in each room, read our guide Watts & Lumens: How much light do I need?

Light bulb shapes

Now you know which light bulb cap you need, you need to decide which bulb shape you want to get.

Light bulbs come in all shapes and sizes, and multiple shapes can be used with numerous fitting types. For example, a classic-shaped light bulb and a candle-shaped bulb can both be found with standard bayonet cap fittings, so this stage is in part down to preference and also in part down to the style, shape and size of your light fitting. Different bulb shapes also create different light spreads to give you different illumination levels, so consider what you want to illuminate and what kind of light you want while deciding on bulb shape.

Note that the names for light bulb shapes are not universal in the same way as the cap fittings are, so there may be some variation between brands.

Not sure which light bulb shape you need? Here are the most common light bulb shapes you can purchase; if you still have your old bulb, simply match it up to the closest shape in our guide below.

GLS or classic bulb

The classic bulb or GLS (General Light Service) is the original bulb design, still the most widely used. It can be used in various light fixtures and interior design styles, from lamps to ceiling lights. The GLS is a classic shape, offering an all-around general ambient light.

Pear-shaped bulb

Pear-shaped bulbs are similar to the classic GLS. However, you can get various forms, from stout pears to more elongated ones. The elongated pear-shaped bulbs provide a more antique look, perfect if you want your lighting style incorporated into your interior design.

Candle

Candle-shaped bulbs mimic the shape of a candle flame and offer a general ambient light. Some even have a flourished point to replicate a flickering flame more realistically. Candle bulbs tend to sit in the decorative bulb category. They are best suited to decorative lighting like chandeliers, wall sconces, small table and bedside lamps.

Globe

Globe bulbs are more bulbous than the classic pear-shaped ones you may be used to, but they provide a striking design statement. Use these bulbs to incorporate your bulb into your interior design. You can choose various sizes, and why not choose a lamp that allows your bulb to peep through to show off your flair for interior design? Offering general ambient light, use a globe bulb to create the perfect atmosphere in your room.

Downlight/ spotlight

A downlight bulb is another name for the GU10 bulb. Their small and compact conical shape makes them perfect for recessed downlights, spotlights and track lights, and they provide a concentrated directional beam for highlighting certain areas. Their reflective insides make them super bright, just what you want for this lighting type.

Spiral

The spiral light bulb design is most common for CFL bulbs. The compact size but increased surface area allow more space for the fluorescent coating inside the tube to react with the UV light. Use these on any light fitting, from ceiling lights to lamps for general ambient light.

Tubular

Tubular bulbs are also in the decorative bulb category, mimicking the traditional style; you can now get these classic-designed bulbs with LED technology that emulates the filament wire inside the bulb, offering general ambient light. These come in various sizes and diameters to suit different types of light bulb fittings and your interior design style. These work well on light fittings where you can show off and leave the bulb exposed, like flush ceiling or pendant lights.

Capsule bulb

A capsule bulb is a small cylindrical or pebble-shaped bulb, usually with a bi-pin base for a G4 or G9 light fitting installation. Traditionally, these bulbs would have been halogen, but LED technology makes them much more energy-efficient and longer-lasting. They are known for their bright, focused light yet come in a small capsule form, so they are common in desk lamps, under-cabinet lighting, pendant lighting and some chandeliers.

Corn bulb

The corn bulb is quite recognisable, named after the look of a corn-on-the-cob due to the small LEDs arranged around its cylindrical body. It comes in various sizes and screw bases, including E26, E27 and bi-pin bases. The cylindrical design offers omnidirectional light when you need wide-angle illumination.

Golf ball

A golf ball bulb sounds like what it is: a bulb in the shape of a golf ball. These small, rounded bulbs are compact and cute, working best in small light fittings like table lamps for all-around general lighting.

Light bulb technology

Do you know which light bulb technology you should use for your light fitting? The four most common types of light bulb technology that most people are familiar with include:

• LED (Light Emitting Diodes)
• Incandescent
• Halogen
• Energy-Saver / CFL (Compact Fluorescent Lamp)

What are LED light bulbs?

LED (Light Emitting Diode) light bulbs are highly energy-efficient bulbs that produce light using semiconductor technology. When an electric current passes through the semiconductor material, it emits light as photons.

Pros and cons

Unlike incandescent or CFL bulbs, LED bulbs do not rely on heating a filament or using gases to produce light, making them far more energy-efficient and durable. LEDs use up to 80-90% less energy than incandescent bulbs and 50% less than CFLs. They also last 15,000-50,000 hours, saving you the most money on electricity and replacement bulbs. LEDs also instantly light up, unlike CFLs.

LED light bulbs are your best option when replacing bulbs. However, If you’ve not used them before, switching to LED bulbs is easier than you might think. Most halogen and incandescent light cap fittings are available as retrofit LEDs to help you make the switch.

What are incandescent bulbs?

Incandescent bulbs are the original light bulb types first invented by Thomas Edison. The filament heats and emits light by passing electricity through a filament wire in the bulb.

Pros and cons

However, the filament can reach temperatures up to 2,000 to 3,000°C, meaning the bulb can get extremely hot to the touch. Moreover, these bulbs use considerable energy to create light, releasing most energy in heat. They are about 90% less efficient than LEDs. Lastly, they only last about 1,000 hours, meaning they need replacing often.

What are halogen bulbs?

Halogen bulbs are a type of incandescent light bulb that uses a small amount of halogen gas (such as iodine or bromine) to increase the efficiency and lifespan of the bulb. Like regular incandescent bulbs, halogen bulbs contain a tungsten filament that glows when heated by an electric current. However, the halogen gas helps prevent the filament from deteriorating as quickly, allowing the bulb to burn brighter and last longer.

Pros and cons

Although more efficient than traditional incandescent bulbs, LEDs and CFLs are still more efficient. Similarly, their lifespan is better than incandescent but not as good as LEDs and CFLs.

Incandescent and halogen bulbs have now been phased out and are no longer being manufactured. Some halogen bulbs are still available to buy, however, these will not be around for much longer.

What are CFL bulbs?

CFL (Compact Fluorescent Lamp) bulbs are energy-efficient bulbs that produce light by passing an electric current through a tube containing argon gas and a small amount of mercury vapour. This process generates ultraviolet (UV) light, which then interacts with a fluorescent coating (phosphor) on the inside of the tube, causing it to emit visible light.

Pros and cons

CFLs are about 70-80% more efficient than incandescent bulbs and last much longer, typically around 7,000-15,000 hours, meaning these will save you money on your energy bill, use less electricity, and you won’t have to replace them as often. You can also get CFLs in a range of colour temperatures.

However, CFLs take a few moments to “warm up”, often appearing dim until they reach full brightness. Furthermore, due to the presence of mercury in these bulbs, they need to be disposed of properly or recycled to prevent damaging the environment.

What colour temperature bulb do you need?

As bulbs have come up to date with technology, you can now buy bulbs with various colour temperatures.

The different colour temperatures of light bulbs can alter how a room feels, so it’s an important factor to consider. From creating an ambient atmosphere to providing white clarity, colour temperature can be amended to suit the room’s purpose.

Warm White

The ideal choice for an ambient glow in bedrooms, living rooms and dining rooms, warm white light bulbs will create a relaxing environment throughout your home. Warm white bulbs range from 2,700K to 3,000K.

Neutral light bulbs

Anything between 3,500K-4,000K is considered neutral light, producing light that is whiter than warm white light but not as stark as the bluish tones of cool white light. Commonly used in offices, kitchens, workspaces and garages because it provides a clear, crisp light ideal for focus and ares where productivity is crucial.

Daylight bulbs

A daylight temperature bulb will range from 5,000-6,000K and is the closest to mimicking natural light. Perfect for security lighting, these light bulbs will brighten your space with clarity.

Cool White

The cool white temperature is ideal for bathrooms or kitchens, adding a functional bluish-white tone to the room. These bulbs will be 6,500K-8,000K.

Read our guide, Warm White or Cool White Lighting, for more information about the best colour temperatures for your home.

Dimmable or non-dimmable?

Lastly, you need to consider whether or not you want your light fitting to be dimmable. Dimmable light bulbs are great for spaces used for more than one purpose, for example, reading, working or watching TV, as you can set the lights to the exact level of brightness you need.

However, not all bulbs are compatible with dimmer switches, so you need to take care when choosing your new bulb.

The good news is that LED light bulbs are available in both dimmable and non-dimmable versions, so you will always be able to find the right bulb to suit your needs. The packaging will always state when a bulb is dimmable, so always check closely before buying. However, you will need a compatible dimmer switch for your dimmable LED light to work correctly.

CFL energy-saving bulbs are not dimmable. In contrast, the remaining halogen bulbs on the market can usually be used for dimmable and non-dimmable purposes.

For more information about shopping for dimmable light bulbs, read our guide:Can I get dimmable LED lights? – A simple guide to dimmable LED bulbs.

Light bulbs at Moonlight Design

You are now equipped with everything you need to know about light bulbs to help you buy the right ones, including light bulb fitting types, shapes, technology, brightness, colour temperature and dimmable bulbs.

The next step is to browse our extensive range of practical and decorative light bulbs at Moonlight Design and find the perfect ones for your fittings.

If you’re still unsure about which of the different light bulb bases you need for your fittings, simply contact the Moonlight Design team on 020 8925 8639 or email us at info@moonlightdesign.co.uk. We’d be more than happy to help.

Related blogs:

• LED Lights: The Pros and Cons
• Lumens Vs. Watts: What’s the Difference?
• What are the different types of lighting?