When you find yourself shopping for light bulbs online, and you find yourself looking at different base screw fittings on your computer, it may be hard to tell whether you are getting the right one. It can be quite confusing to know which screw or bayonet cap is the right size. It can get even more confusing when you check the screw caps, and you notice that there are only a few millimeters between the sizes.

Don’t worry, once you understand the meaning of the different cap types, you can order your bulb without any worries. All you need is your trusty measuring tape. You can browse through our online shop, and you can check out product descriptions or specifications.

The names for screw-in fittings start with the letter ‘E’, which means Edison Screw. The second part of the name corresponds to the diameter of the lamp holder into which the bulb screw goes into. So, E12 screws fit into a 12mm holder while an E17 screw fits into a 17mm diameter holder, and so on.

Take note that the number measures the diameter of the lamp holder and not the size of the bulb’s screw. This is because the screw cap on a bulb is slightly smaller than the fitting. If these were of the same size, the screw wouldn’t fit.

What is Edison Screw?

Edison Screw is a standard light bulb socket for most electric light bulbs. Thomas Edison developed this socket, and it was patented in 1881. These bulbs feature right-handed metal bases, which fits into the lamp holders’ matching threaded sockets.

The thread is connected to neutral for bulbs that are powered by AC. The bottom tip of the base is connected to the live phase. In Europe and North America, the Edison Screw has displaced other types of sockets.

5 Common Thread Size Groups

• Candelabra, which is E12 in North America (12MM diameter)
• Candelabra E14 in Europe (14MM diameter)
• Intermediate, which is E17 in North America (17MM diameter)
• Medium or Standard, which is E26 in North America and E27 in Europe (26MM-27MM diameter)
• Mogul, which is E39 in North America and E40 in Europe (39MM to 40MM diameter)

E12

E12 has a diameter of 12 mm, E17 has a diameter of 17 mm, E26 has a diameter of 26 mm, and E39 has a diameter of 39 mm. Most light bulbs in the United States are E26, and it is commonly referred to as “medium” or “standard” base.

E12 is a small candelabra base. It is used for nightlight bulbs, as well as decorative lightbulbs for chandeliers and bathroom mirrors. It is found in America, especially in old or imported fittings. However, it is not common, so you will hardly see stocks of this fitting in your local supermarket. But you can still find a range of this fitting with specialist suppliers.

E17 is the intermediate base, and it is found between two sizes, E12, and the common E26. This fitting is often used for desk lamps and appliance bulbs, but they are not common. The larger of these four fittings, the E39 or the mogul base is typically used in street lights as well as high wattage lamps. They are uncommon, and you won’t likely have a need for this fitting.

You will find other screws that exist for other uses. Some of these screws include the following:

E10

This screw is oftentimes referred to as MES or Miniature Edison Screw. These things are often used in old chandeliers. The bulbs designed for these screws and lamp holders are typically more decorative in appearance. You will find E10 bulbs in most miniature lamp applications.

E11

This is often referred to as mini candelabra. It is 11 mm in diameter, and it is a bit smaller than E12 candelabra, which is about 12 mm in diameter. E11 screws are often used in small halogen bulbs, and E12 are used for decorative lights. You can use these adapters to fit new LED lights into old halogen fixtures.

E14

This is referred to as SES or Small Edison Screw. This is the most common screw size in the United Kingdom and throughout Europe. You can find it in most small light fittings around the home. They’re used in bedside table lamps to wall lights and even chandeliers. E14 bulbs come in various shapes including candles, golf balls, spotlights, and filament bulbs. You just need to know which shape would best suit your screw fittings.

E26 and E27

E26 is the standard screw size for most large mains voltage light fittings in the United States. It’s not used in delicate light fittings like chandeliers. It cannot be used for old fittings as well. Of course, there are exceptions to this. However, there is a wide range of bulb shapes available for E26 screw fittings. An example would be a PAR20 halogen bulb replacement which is a reflector flood bulb.

While E26 is the standard 120-volt American base, the E27 is Europe’s variant. It is rated at 220 volts. E26 is 26mm in diameter, and E27 is 27 mm in diameter. An E26 bulb can fit into E27 and vice versa without any issue. These sockets are interchangeable, but they have a different voltage rating.

E39 and EX39

An EX39 fitting has a longer tip at the screw base. It can work with both standard E39 and EX39 lamp sockets. The bulbs for EX39 bases typically come with protective shields, so they can be used for both open or closed fixtures. However, E39 bulbs will work with E39 lamp sockets, but it will not work in an EX39 socket. The E29 bulbs are used in enclosed fixtures only.

Like E26 and E27 bulbs, E39 bulbs are interchangeable with the E40 because there is only a 1 mm difference in their diameters. These fittings are used for streetlights, high-intensity discharge lamps, and high-wattage lamps, usually with 100W, 200W, and 300W bulbs. Regular lamps with over 300W are not allowed to use an E26 base. You need to use the E39 base following the National Electrical Code.