Light Bulb Fittings Explained: ES, SES, Bayonet, GU10 and G9
Almost everyone has done it. You bring a new bulb home, climb the stepladder, offer it up to the fitting and discover that the two were never going to go together. Light bulb fittings are one of those small details nobody thinks about until the moment they matter, and picking the wrong cap is easily the most common reason a bulb ends up back in its box.
The good news is that British homes only really use a handful of fittings. Once you can recognise them on sight you will never buy the wrong one again. This guide takes each type in turn, explains what it looks like and where it usually turns up, then shows you the quickest way to identify whatever is already sitting in your lamp.
Why the fitting matters more than the bulb
The fitting, sometimes called the cap or the base, is the part of the bulb that connects to the lamp holder. Almost everything else about a bulb is a choice you are free to make. Brightness, colour temperature, shape, finish and whether it dims are all flexible. The cap is not. It either fits the holder or it does not, and there is no middle ground.
Manufacturers describe caps with a letter and a number. The letter tells you the mechanism, so E means an Edison screw, B means a bayonet and G means pins. The number is a measurement in millimetres, either the diameter of the screw thread or the distance between the pins. An E27 is a 27mm screw and a GU10 has pins set 10mm apart, which is all the code is really saying.
Britain is more mixed than most countries because we adopted the bayonet cap early and then gradually took on screw fittings alongside it. That history is why a single house can easily contain four or five different caps, with bayonet on the landing, screw in the table lamps, GU10 in the kitchen ceiling and a few tiny capsules in the bathroom.
ES and SES: the screw fittings
ES stands for Edison Screw and is written E27 on the packaging. It is the larger of the two common screw caps, measuring 27mm across the thread, and it goes in with a simple clockwise turn. You will find it in most pendant lights, the majority of floor lamps and any table lamp of a reasonable size. If a bulb looks like the classic household shape, it is very often an E27.
SES is the small version, written E14, with a thread 14mm across. It suits anything on a smaller scale, so think candle bulbs in a chandelier, wall lights, bedside lamps and the little accent fittings that go on a shelf or in an alcove. The two screw sizes are different enough that you will spot it immediately once you have one of each in front of you.
There is also a giant screw cap called GES or E40, but it belongs to street lighting and industrial high bays rather than homes, so you are unlikely to meet it. For everything domestic, our ES bulbs cover the standard screw size in LED, filament and frosted finishes, and the small screw versions sit alongside them in the same range.
BC and SBC: the bayonet fittings
BC means Bayonet Cap and appears as B22 on the box. Instead of a thread it has a smooth metal base with two small pins sticking out of the sides. You push the bulb into the holder, twist it a quarter turn clockwise and it locks in place against a pair of springs. For decades this was the standard British fitting, so it turns up constantly in ceiling roses and older lamps.
SBC is the smaller relative, written B15, with a 15mm base. It appears in smaller wall lights, some older candle fittings and a fair number of appliance lamps. It is far less common than it once was, but plenty of much loved older fittings still take one, so it is worth knowing the name before you go looking for a replacement.
Bayonet caps have one genuine practical advantage in that they lock rather than rely on friction, so they cope better with vibration and with fittings that hang. Screw caps can work themselves slightly loose over years in a ceiling fitting. Adaptors that convert between the two do exist and sit with our lighting accessories, though they add length, so always check the bulb will still fit inside the shade.
GU10 and MR16: the spotlight fittings
GU10 is the fitting in almost every recessed downlight and bar spotlight installed in this country over the past twenty years. The bulb has a flat, chunky face with two stubby pins on the back, set 10mm apart, each ending in a small disc. You push it up into the holder and twist, much like a bayonet, and it runs directly off mains voltage with no extra equipment involved.
MR16, properly called GU5.3, looks similar from the front but has two thin straight pins that push in rather than twist. The important difference is that it runs on 12 volts, which means there is a transformer or LED driver hidden in the ceiling. The two are not interchangeable, so the pins are the detail to check. Stubby pins with discs means GU10, plain thin pins means MR16.
With spotlights the beam angle matters as much as the brightness. Something around 36 degrees is a sensible general choice for a kitchen ceiling, while a narrower 24 degree beam is better for picking out a worktop, a painting or an island. Mixing wildly different beam angles along the same run of downlights is the usual reason a ceiling ends up looking patchy.
G9 and G4: the small capsule fittings
G9 is a tiny capsule with two looped wire pins set 9mm apart that simply push straight into the holder. It runs on mains voltage and has become very common in modern chandeliers, bathroom fittings, small pendants and any design where the maker wants a compact source hidden inside glass or crystal. LED versions now do the job at a fraction of the heat the old halogens produced.
G4 is smaller again, with two thin straight pins 4mm apart, and runs on 12 volts through a driver. It is the classic under-cabinet and display cabinet capsule, and it appears in some small decorative fittings too. As with the spotlights, the voltage is the thing to get right, because a mains capsule pushed into a low voltage holder will not work as intended.
One point that catches people out is that plenty of chandeliers do not use capsules at all and take small screw candle bulbs instead, so check before you order a set. If yours takes the screw version, our SES bulbs cover the candle shapes in clear and opal. With any capsule, check the length as well as the cap, since a millimetre or two decides whether the shade still closes.
How to identify the fitting you already have
The quickest method by far is to take the old bulb out and read it. Nearly every bulb has its cap code printed on the base or around the glass, usually next to the wattage and the voltage. If you can see E27, B22, GU10 or G9 written there, the job is done and you can buy a replacement with complete confidence.
If the bulb has broken or the printing has worn away, measure instead. For screw caps, measure the widest part of the thread, where 27mm means ES and 14mm means SES. For bayonet, measure across the smooth metal base, so 22mm is BC and 15mm is SBC. For anything with pins, measure between the centres of the two pins and note whether they twist or push.
When the bulb is gone entirely, look at the holder itself. A spiral groove inside means a screw cap, two slots cut into the side mean a bayonet, and a pair of small holes means pins. Photograph it against a ruler if you are unsure. Our showroom team identify caps from customer photographs most weeks, so it is always worth asking rather than guessing.
Choosing the right bulb once you know the fitting
With the cap settled, everything else is a matter of preference. Decide the brightness in lumens rather than watts, since watts only tell you what a bulb costs to run. Pick the colour temperature next, where 2,700K warm white suits living rooms and bedrooms and 3,000K works well for kitchens and bathrooms. Finally, if the fitting is on a dimmer, make sure the bulb itself is marked as dimmable.
Shape matters whenever the bulb is on show. Open pendants, glass shades and anything without a cover are the places to use a decorative bulb rather than a plain one, and our white LED filament bulbs give you the look of an old fashioned filament with none of the running costs. Where the bulb is hidden inside a shade, a simple opal is usually the better and cheaper answer.
It is also worth buying a spare or two while you are at it, particularly for chandeliers and multi-arm fittings where a single dead bulb spoils the whole thing and a later batch may not match. You can browse the whole range in our light bulbs collection, and if you are still unsure which cap you need, send our team a photograph and we will identify it for you.
Frequently asked questions
What are the most common light bulb fittings in UK homes?
Five cover almost everything. ES or E27 is the standard screw cap used in pendants and larger lamps, SES or E14 is the small screw cap used in candle bulbs and wall lights, BC or B22 is the push and twist bayonet found in ceiling roses, GU10 is the twist-lock spotlight cap in downlights, and G9 is the small mains capsule used in modern chandeliers and bathroom fittings.
How do I tell the difference between ES and SES bulbs?
Measure the widest part of the screw thread. ES, written E27, measures 27mm across and is the larger household size used in pendants and floor lamps. SES, written E14, measures 14mm and is used for candle bulbs, chandeliers and smaller table lamps. The difference is obvious side by side, and the code is usually printed on the base of the bulb.
Can I use a screw bulb in a bayonet fitting?
Not directly, because the two mechanisms are completely different. You can buy a small adaptor that converts a bayonet holder to take a screw bulb or the other way round, and these work perfectly well. Bear in mind that an adaptor adds around 20mm of length, so check the bulb will still sit comfortably inside the shade before ordering one.
What is the difference between GU10 and MR16?
Look at the pins. GU10 has two stubby pins that end in small discs and twists into place, and it runs on mains voltage with no extra equipment. MR16, also called GU5.3, has two thin straight pins that push in, and it runs on 12 volts through a transformer or driver. They are not interchangeable, so match whichever your fitting already uses.
How do I find out which bulb my light takes if the old one has broken?
Look inside the holder. A spiral groove means a screw cap, two slots cut into the sides mean a bayonet, and small holes mean a pin fitting. Measure the diameter of the holder or the spacing between the pin holes to confirm the size. If you are still unsure, photograph the holder next to a ruler and ask our team, who identify fittings from photographs regularly.
