A Guide to Light Bulb Fittings and Sizes

We’ve covered the shapes of light bulbs in a previous article, but while the shape is partly aesthetic, the fitting is purely functional. This is the part of the bulb construction that physically connects to the electrical contacts, so has to mirror the shape of the housing perfectly. The fitting is normally also the means of securing the bulb to the fitting, but in some cases there’s also another means, such as a clip, to hold the bulb and the housing together.

Naming conventions

When you’re shopping for light bulbs, you’ll see codes for the different sizes. While they might look arbitrary and confusing at first, they do usually have a meaning.

If there’s a number, for example, it will generally refer to a size, in millimetres. That could be the diameter of the socket (as in B and E type bulbs), or it might be something else, such as the distance between the pins (as in the GU10, which has pins 10mm apart).

The letters usually refer to the family of bulbs. B stands for bayonet fitting – that’s where there’s a locking device that works using pins protruding from the sides. It got its name from the similarity to the means of attaching bayonets to rifles, and has two electrical contacts on the base. E comes from the fitting’s inventor, Thomas Edison. This is a screw thread type, where there’s one electrical contact on the base, and the screw thread itself is the other contact.

Common fittings

Over the years, there have been countless bulb fitting designs, but there’s always been one important limiting factor. Bulbs need to go in sockets, so the manufacturing industries needed to standardise the sizes so they could remain commercially viable.

That’s how we’ve ended up with a hardcore of popular fitting sizes, namely B22, E14, E27 and GU10 bulbs, which make up the vast majority of bulbs sold today. Other once-popular types, such as fluorescent tubes, are now steeply declining in popularity as LEDs have become the standard. We’ll be covering some moderately common sizes here, but not the specialist ones.

Edison screw fittings

The E family refers to the screw fitting bulbs. Most common are E27 and E14, but there’s also an intermediate size, E17.

E27 bulb fittings

E27s have a screw thread with a diameter of 27mm. They are a standard fitting for large teardrop, globe and reflector lights, as well as legacy CFL bulbs. They’re common as the main light(s) in a room as they provide a solid connection that can hold quite heavy bulbs. It can be quite tricky to find smaller bulbs, such as candle bulbs, with this fitting, but they are relatively available.

E17 bulb fittings

E17s are not particularly common, and you’ll have to find niche suppliers for them. Not many appliances are designed for them.

E14 bulb fittings

The smaller Edison fitting, E14, is very common. You’ll find this in table lamps, desk lamps, feature lighting, chandeliers and all manner of things around the home. It’s a screw thread fitting with a 14mm diameter.

Other E sizes

Think of a number and there’s probably an E of it. There are E40s for demanding tasks like warehouse lighting, where the bulbs need to be large, and E5s for small applications such as Christmas decorations. They are only sold by specialist bulb suppliers.

Bayonet fittings

Bayonet (B) fittings are another standard bulb fitting. You push the bulb into the socket, twist it by 90 degrees and it’s held in place by a spring-loaded housing and a slight kink in the groove. 

B22 bulb fittings

B22 bulbs have a base with a 22mm diameter, and a small pin on each side. These are incredibly common, and tend to be for larger bulbs. 

B15 bulb fittings

The B15 has a 15mm diameter base with two pins, just like the B22. Its smaller size makes it appropriate for table and desk lamps, wall lighting and a host of other applications where a smaller construction is used. Most modern devices tend to use an E15.

Pin fittings

With pin fittings, there are two prominent pins that protrude from the base. They need to be pushed into the housing to make the electrical connection. Some have built-in locking devices similar to the bayonet, others require a secondary means of securing the bulb in place, and some smaller ones can be held in place simply by the friction of the housing connections.

GU10 bulbs

The GU10 is a parabolic bulb with a two-pin fitting at the base. The pins are 10mm apart and have widened ends that lock the bulb in place when twisted by 90 degrees. They are used in downlighting as they can be embedded into the ceiling or the underside of a cabinet while remaining relatively low profile.

GU5.3 fittings

The GU5.3 has two straight pins that are 5.5mm apart, and they push directly into the housing. As there’s no locking device built in, they will often need to be secured in place using a clip or another type of secure housing.

GX53 fittings 

A relatively niche pin fitting is the GX 53. It’s an extremely low-profile bulb because the pins are at the sides of the bulb (53mm apart) rather than behind it. The drawback is that this makes the overall diameter of the housing relatively large. However, if it’s being used for downlighting below eye-level, such as being mounted on the underside of kitchen cabinets to illuminate a worktop, there’s no aesthetic cost.

Tube fittings

Although they are on the road to obsolescence, fluorescent tubes are still in use, largely because they still exist in the millions in the ceilings of factories, workshops, kitchens and shops. They can be secured in place either by a twist function or via a spring-loaded end component.

G13 caps

This is the standard fluorescent tube that anyone over 40 no doubt remembers well. They have two pins at both ends, 13mm apart. They’re sometimes called T8s, which refers to the diameter of the tube itself (8/8 of an inch, or 1 inch), rather than the pins.

G5 caps

G5 caps have two pins spaced 5mm apart. They are common in smaller fluorescent tubes, but also in shaped tubes for smaller applications, such as inside fridges or in wall-mounted bug-zappers.

S15 caps

This is a second type of tube fitting, where there’s a solid 15mm connector cap at each end. They tend to be for smaller applications, such as shaving mirrors.

Small fittings

There are also some small bulb fittings that are useful for back-lighting, decorative lighting or lighting the insides of cabinets. They have a simple two-pin construction but they are so small and light that they don’t need any extra means of securing – a slightly grippy housing contact will do the trick.

G4 bulb fittings

G4s are tiny 12V bulbs with two protruding contacts 5mm apart. You’ll find them in small appliances or decorative pieces, but they also have many uses in the automotive industry, where they are used as backlights for gauges and interior lighting. Despite their small size, they can kick out a fair amount of light, so can be used for kitchen worktops.

G9 capsules

G9 capsules are small 240V lights, notable by their twin looped contacts below the base. The capsule features a flattened base, which assists in gripping the bulbs in their housing. They have similar applications to G4s above, but there’s no need for a 12V transformer.