How Do I Know What Base My Light Bulb Is?

How many hourly electricians does it take to change a light bulb? Two: one to hold up the bulb, and one to spin the ladder around.

Regardless, getting the correct light bulbs to install into your light fixtures is not a very difficult task. All you need is a ruler and this nifty guide.

To know the base of a light bulb, identify if it is a screw type that has threads around the base, or a pin type that has 2 points of contact on the base. After that, measure the diameter of the screw base, or the distance between the two pins, in millimeters.

Light Bulb Bases Explained

Interestingly the most common type of bulb is called the Edison base, a homage to the creator of the original light bulb, Thomas Edison, which used E26 screw type, medium size light bulb base.

These E26 bulb bases are used in general applications such as ceiling lights, lamps, ceiling fans, and many other places around the home and office.

They are found in bedrooms, living rooms, and, if used with reflector bulbs, spotlights and floodlights.

One of the cool facts about light bulb bases and the codes they use is that they all mean something important!

All bases are measured in millimeters. Thank God for Edison sticking to the Metric system of measurement even though he was an American inventor. And the E is all the screw type bulbs stand for, you guessed it, ‘Edison.’

So the E26 measures exactly 26 mm across the diameter of the base of the bulb.

The E12 screw type bases are called candelabra, chandelier or pendant lights. They are also common as they are used in decorative chandeliers and table lamps, or wall sconces, festive lights, restaurant fixtures, and marquees.

They give off a medium to low luminance depending upon the wattage and lumens and create a cozy and festive mood around the area.

After screw bases, we come to the other standard type of base, called pin type. These bases are famously found in fluorescent tubes and compact fluorescent lights, specifically the bi-pin G13.

These bases are majorly found in older-style kitchens, garages, and office settings. Again, the 13 indicates the distance in mm between the pins. The G refers to Glass, the material used in the original tubes.

Among these pin bases, the leader is the GU10 bulb, another very commonly used base, and it is a pin type base, which runs on 120 Volts. These are familiarly known as spotlights! You can guess from the name where they are most commonly used.

You will find them in the downlights and recessed can lights around a home, especially in the kitchen. They are also commonly used in track lights and also in garden spotlights.

So essentially, GU10 spotlights are found in shops, retail spaces, indoors and outdoors due to their wide application.

The pins or prongs of the GU10 are actually small circular knobs, 10 mm apart, and are plugged in using a twist and lock mechanism, which forms a very secure connection.

However, don’t mix them up with G9 bulbs as these are entirely different. The G9 bulbs often get stuck due to their hook type base.

Finally, you have the bayonet type bases that are basically raised bumps on the base to allow contact, and can be either single or double contact bayonets. They are coded as BA15d or BA15s.

Different Light Bulb Base Sizes Chart

Let’s take a look at the most common light bulb base sizes across the three main bulb base types.

Starting with Edison screw base bulbs, there are four main sizes, although these vary slightly between the US and Europe.

The most common is the E26 in the US, and the E27 in Europe. However candelabra bulbs (E12 or E11) are also fairly prolific. The intermediate and mogul sizes are a little more rare.

E14 are used in Europe as night lights or in selected wall lights, while E17 in the US often have applications in domestic appliances, such as refrigerators.

Edison Bulb Sizes	North America (base width)	Europe (base width)
Candelabra bulb	E12 – 12mm	E11 – 11mm
Intermediate	E17 – 17mm	E14 – 14mm
Standard (Most Common)	E26 – 26mm	E27 – 27mm
Mogul	E39 – 39mm	E40 – 40mm

When it comes to twist and lock bases, there are two main base types – bayonet and spotlight.

Bayonet bases aren’t too common in the US, but they’re fairly prolific across Europe.

While both the BA15 and B22 light bulb base types are found in both regions, the BA15 is very slightly more used in the US while the B22 is quite well known across the pond and used in many different lighting fixtures.

With twist and lock spotlights, the GU10 is standard everywhere.

Twist and Lock Bulbs	North America	Europe
Bayonet	BA15 – 15mm between pins	B22 – 22mm between pins
Spotlight	GU10 – 10mm between pins

When it comes to pin base lights, there are quite a few different kinds. The most common are the G5, which are used in CFL tube lights, and the G5.3 which are often used in spotlights. As with all other bulbs, the number refers to the base width in milimeters.

A lot of these light bulb base sizes are common in LED bulbs and have many uses including festive lights or other decorative lighting for your home.

Pin-bulbs	Width in mm between pins	Bulb use
G4	4 mm	Wide-angle beam, used in lamps some chandeliers
G5	5 mm	Common size for tube lights
G5.3	5.3 mm	Standard spotlight size
G9	9 mm	Looped prongs, mainly used under cabinets
G12	12 mm	Mainly used to replace metal halide lamps
G13	13 mm	Common larger size for fluorescent lights

Why Do Fittings Have Different Bases?

Different bases serve different purpose.

Screw bases had been the standard for years, as the connection is secure and stable. The bulb can withstand frequent vibrations in areas where doors open and close, or appliances run, without falling out or becoming loose.

Pin type bases allow a more compact socket, so smaller bulbs can fit into a smaller fixture or light much more easily. More decorative and modern lighting streamlined in shape, requiring a more efficient way to fit a bulb.

The twist and lock mechanism is also much faster and easier to perform than tediously screwing in Edison lights in a tight spot.

The base or cap provides the electrical point of contact between the bulb and the electricity. Also, it helps to secure the bulb in place inside the socket.

You can, in fact, safely change SOME bases by using adapters that convert the base to the socket type that you have.

For example, you can buy adapters where you can screw in an E12 base, and it ends in an E26 base, which you can then screw into your corresponding socket.

Similarly, there is an adapter for converting a GU10 bulb to the Edison base. You can push in the pin base into the adapter, which can then be screwed in.

A huge word of caution at this point. Like other adapters mentioned above, you can sometimes see online sellers selling adapters converted from an Edison screw bulb to a bayonet cap base.

In this case, as you screw in your light bulbs into the socket using an adapter and accidentally touch the base’s metallic part, you could receive a lethal electrical shock.

This is because the double bayonet base has introduced polarity to the Edison base, and is no longer neutral and safe to touch while plugging in. Under normal use, even the metallic part of the base in an Edison bulb is safe to touch as it is neutral.

It is not recommended to purchase this adapter in any case.

What Is Standard Base Type?

The most standard bulb base used all over the world and specifically in the US is, of course, the E26 screw-type base. You can never go wrong with them, and it is always a good idea to have some in stock in the house.

This base is very similar to the E27 commonly sold in Europe and Asia. It is only a millimeter bigger than the American standard.

They can both be used interchangeably, provided their voltage is the same or is a universal bulb.

Does Bulbs Base Affect Voltage And Performance?

Some bases allow only line voltage bulbs to be connected, so in the US is 120 Volts. Those are mostly the E light bulbs.

Some bases allow only low voltage bulbs to be connected, which are only 12 V, or multiples (24 V, 36 V). These are found in small spotlights in pin-type bases, used in aquariums or small decorative pieces.

You cannot interchange these as they will either not work, or become permanently damaged owing to the higher or lower than intended voltage passing through.

Other than that, the bulb base has nothing to do with the voltage the bulb runs on, nor does it alter the luminance, wattage or color temperature of the bulb.

Final Words

The Edison screw-type light bulbs are not going anywhere soon. While conventional incandescent bulbs and CFLs are slowly being phased out, the screw base is here to stay for the foreseeable future.

Now that you know how to identify and measure the base of your light bulb, or the corresponding socket, you might be able to answer it.

How many psychiatrists does it take to change a bulb?

One, but the bulb must want to change.

So which bases do you have around your home or office, and in which areas?