Standard Light Bulb Base Sizes

You’ve built the home of your dreams, but now the lights aren’t coming out. 

You’re getting ready to take your travel trailer on the road, but now the light bulbs aren’t working.

Whether it’s your permanent home or home on wheels, you need to have the lights on. 


The biggest differences between E26, A19, E27, A21, A15, BR30, ST19, & E12 are the diameters of the base and the light bulb. The difference between A19 and E26 is that E26 refers to the base, and A19 to the shape of the light bulb.

In this article, we’ll also cover the following topics:

  • The different bases
  • The different shapes

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light bulb

You’ve probably heard that you need a type-A light bulb, but are you sure that it’s a standard base you’re dealing with? 

It just so turns out that there are different socket bases, depending on what you’re dealing with. Did you know there’s also such a thing as a type-T?

We’ll teach you everything you need to know about the topic. You’ll learn about standard light bulb bases, and we’ll even provide you with some decorative options to consider. 

How does that sound? We’ll get you on the road with the well-lit evenings you deserve, whether for a cozy night or a night of book reading.

A19 light bulb

Table of Contents

Standard light bulb base sizes

While there’s a range of options to choose from, the E26 light bulb base size is by far the most popular in the US. The second most popular option is the E12 Candelabra bulb base.

In their respective sections, we’ll take a look at each. However, the E12 is usually used for more decorative settings. E26 is by far the more normal option.

The good thing from a compatibility point of view is that you won’t have to concern yourself with whether it’s incandescent or LED. It’s really the socket size that will determine whether you can use it. 

Both incandescent and LED E26 will have the medium base socket size you are looking for. It doesn’t matter that the way they work isn’t the same. 


ST19 light bulb

Here’s a quick light bulb size chart summary that will hopefully make everything easier to understand. This article explains it thoroughly, so you should now know which socket you’ll need.

Socket diameterMarket

While it may summarize a lot of information, it’s still better to ensure you get the right lighting situation wherever it’s needed. It will help both accomplish the intended need and set the mood!

decorative light bulb

However, let’s dig a little deeper into the subject. Let’s take a look at some of the specific models to see how they differ. What makes one a better choice than another? 

This way, you’ll be able to get the best light solution possible.

E26 bulb

E26 light bulb

As we’ve mentioned, it’s the most popular option in America. It’s the most regular and standard option there is. This specific type is referred to as having a medium or standards base, and the “E” refers to its shape. 

In fact, the “E” stands for Edison screw. The number is a reference to the thickness of the base. The E26 has a base that’s roughly 1 inch in diameter or the equivalent of 26mm. 

If a bulb has the same diameter and screw as outlined in this section, it will be referred to as an E26. When you pick up a bulb whose base is roughly an inch in diameter, you’re likely dealing with an E26.

It’s also typically indicated on the packaging despite being the market’s most common model. It’s important that the classification does not refer to the shape of the bulb but rather the base. 

A19 bulbs

When you’re going to Home Depot, and you’re looking around the electrical aisle, there are several things you will see. 

You will probably see the term “A19” popping up. That’s no surprise. By the end of this section and the next, you’ll know the differences between A19 and E26.

A19 refers to the shape and dimensions of the light bulb rather than its base. It has long been the golden standard used for residential and commercial purposes. 

If you’ve ever thought about the classic light bulb shape, this is probably the one you’ve thought of. You will see a striking resemblance when you put it next to an upside-down pear.

So, what’s the number that follows the “A?”

It refers to the diameter of the bulb. It’s measured at the widest point. Here’s the tricky part. It’s measured in eights of an inch rather than millimeters. 

How thick do you think it is? It’s usually constructed to be 2.4 inches. It’s the equivalent of taking an eight of an inch 19 times. You get 2.4 inches by dividing 19 by 8.

Countries that use the metric system will have different names for this light bulb. Rather, they will call it an A60. The “60” refers to the diameter the same way that the “19” in “A19” does. The unit of measure is just different. 

You will typically find that A19s are about 100-110mm in length. It’s the equivalent of 3.9-4.3 inches. 

A19 is a standard that is governed by the ANSI c79.1. Standard dimensions have been determined and will vary depending on the different wattages you’re dealing with. 

It’s generally the case that A19 cannot be longer than 112.7 mm in length. At the same time, it cannot have a diameter greater than 69.5mm. 

Underneath, you’ll find a chart that easily shows you the maximum measurements for the different wattages of an A19. 

40W Type 210:40W Type 211:50W Type 206:50W Type 211:60W:75W:100W:
max length108 mm112.7 mm100 mm112.7 mm112.7 mm112.7 mm112.7 mm
max diameter69.0 mm69.5 mm68.0 mm69.5 mm69.5 mm69.5 mm69.5 mm

A19 vs. E26

A19 vs. E26 bulbs – what’s the deal? If you’ve gotten this far into the article, you may be able to know the difference between the two. The A19 refers to the shape of the light bulb, whereas the E26 refers to the base.

The standards we spoke of earlier have specified some things about the A19, including its base.

Is an E26 the same as A19?

That is the important question. A19 will always use the E26 base. However, an E26 base does not need to have the shape of an A19 light bulb. 

Do you understand it? One is always the other, but not necessarily the other way around. E26 is not necessarily the same as A19 because they measure different things. 

As the E26 is the most common base used in America, you are generally good if you get a light bulb marked as A19. 

You’ll also be happy to know that the A19 standards are the same for both incandescent and LED lamps. LED ones have gained significantly in popularity, but the classification needed to remain the same in the future. 

Some advantages of LED lighting include:

  • Design flexibility
  • Energy efficiency
  • Long lifespan
  • No heat or UV emissions
  • Low voltage operation

It allows for easier retrofitting when you’re changing things out. 

E39 sockets

This base is often used on older metal halides and high-pressure sodium. They usually start at 100 wats and go up. As with E26, the letter stands for the screw. It resembles the base of the E26 but with a diameter of 39mm instead.

The diameter of the base allows it to transfer more energy. It means they’re more common in industrial settings. If you think stuff like parking lot lights and warehouse lights, those are places you’ll commonly find them.

They’re also used for wall packs and canopy lights. Whereas the E39 is more popular in industrial settings in America, the norm in Europe is E40. The difference in diameter of 1mm means they can sometimes be used interchangeably. 

In the section on E26 vs. E27, we’ll explain why you only may be able to use them interchangeably.

E12 sockets

Having gone through both the E39 and the E26, you may now have an idea as to what the deal is with the E12 base. 

Again, the “E” from the E12 refers to Edison. The number refers to the diameter in millimeters.

Since both the E26 and the E39 can transfer more energy with their larger diameters, it’s obvious to see why the E12 can transfer less. It also means that they’re usually used for different purposes. 

Pendant lights and various types of decorative purposes are usually where you’ll find these. Their European counterparts are the E14s. Those are also used for the same types of decorative purposes. 


E17 has a diameter that lies in between the E12 and the E26. If you find yourself looking for refrigerator or microwave lights, these are relatively common.

Throughout the house, they’re not that common. Your ceiling fan could also be using this type. You may take out the old light bulb and see that it doesn’t seem quite the same size as the E12 or the E26. In those cases, chances are it’s an E17.

E26 vs. E27

E27 light bulb

With there only being a one-millimeter difference in the size of the base, what’s different between the two? Well, it turns out that the E26 is common on the American market. The E27 is the general standard in the European market. 

There’s obviously a mechanical difference in the diameter of the base, but it exists for a reason. 120v AC is the norm in North America. In Europe and a lot of other places, 240v AC is what’s being used. 

They cannot be used interchangeably if there isn’t sufficient clearance in the light bulb. The clearance needs to be between the live and neutral contacts. Without the proper clearance, there’s an increased risk of a short circuit when it’s being used. 

Moisture and contamination pose risks when it comes to electrical fires. In order to counteract that risk, the E27 uses a wider thread trough diameter. Other measures are also taken to avoid a short circuit with a higher voltage. With a taller insulator height, the risk of a short circuit is lessened. 

If these precautions aren’t taken, the E26 and E27 light bulbs may not be used interchangeably. You shouldn’t assume they’re interchangeable when shopping, even if one is only a millimeter thicker at the base. 

The higher voltage in the European market means E26 shouldn’t be used there. On the other hand, E27 may sometimes be used for 120v AC applications. 

With its lessened insulation and protective measures, you’re creating a fire hazard using E26 with 240v AC. For 240v AC, E27 should always be sought out.

What is an A21 bulb?

As with the A19, the term “A21” refers to the shape. Do you remember how the shape of the A19 was determined? 

It has a lot of similarities to the A21. The A21 is simply a little bit bigger in its diameter. Rather than having a diameter of 2.4 inches, the A21 has a diameter of 2.6 inches. 

The A21 bulb is often used in:

  • Table lamps
  • Ceiling fans
  • Wall lights
  • Higher light output applications

It’s the equivalent of 21 divided by 8, as the number indicates the number of eights in diameter. The equivalent of an A19 was A60 in countries that use the metric system. The equivalent of the A21 is A67, which refers to the fact that it has 7 more millimeters of diameter than the A60.

100W (v 215)100W (v 216)150W (v 262)
max length112.7 mm 134.9 mm139.7 mm 
max diameter75.5 mm78.0 mm78.5 mm 

A21 uses the same standard for socket size by using the E26. Once again, an E26 can have the shape of an A21 but can also use a different socket size. An A21 will always use an E26.

A21 vs. A19

The A19 is slightly shorter than the A21 bulb. The A21 bulb will typically have a length between 105 and 140mm. 

ResemblesUpside-down pearUpside-down pear
Largest length112.7 mm139.7 mm 
Largest diameter69.5 mm78.5 mm

A15 vs. A19

A19 is a bit bigger than the A15 light bulb. Once again, the letter refers to the diameter. The A19 has a diameter that is ½ inch bigger than the A15. With their measurement method being the same for the two, you can find the A15’s diameter by dividing the number by 8. 

It means the diameter comes out at 1.875 inches. The A15 also uses the E26 socket, which is important when replacing it. 

BR30 vs. A19

The BR in BR30 stands for “bulge reflector.” The main difference between the two involves their shapes and how they disperse light. With its pear-like shape, the A19 disperses light evenly across the room, which is different from the BR30. 

With its squished shape, the BR30 disperses light straight ahead instead. 

When buying a home bulb, you will want to go with the A19 because of how it disperses lights. The BR30 will seem much stronger in some areas and a lot weaker in most other places. BR30 is great for places where you want directional light, but that is typically not the case in the middle of your living room. 

However, both are standardized shapes that serve their intended purposes well. They may not serve the same purposes equally well.

BR30 vs. A19 ST19 vs. E26

Edison light bulb

Both BR30, A19, and ST19 use the E26 socket. The question becomes what they’re best used for. 

The ST19 is the vintage Edison-style, whereas the BR30 looks squished, and the A19 looks like an upside-pear. When it comes to decorative purposes, the BR30 isn’t great. However, you can use both A19 and ST19 for decorative places. 

Used forDirectional lightMulti-purpose, including home & decorativeDecorative
ShapeSquishedUpside-down pearVintage Edison lamp

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