What Is The Difference Between A19 And A21 Light Bulbs?

Are you confused by all the different codes when buying a new light bulb for your home?

There are many options, and if you don’t know your light bulbs well, you might not be sure about which bulbs you can buy.

A19 and A21 sound really similar, though, so maybe they’re pretty much the same bulb? Do you need to worry about choosing between them?

The difference between A19 and A21 bulbs is their size. A19 bulbs measure 2.375 inches in diameter, while A21 bulbs measure 2.625 inches. Both usually have the same E26 Edison screw base, although, in Europe, some older B22 bayonet bulbs may still be used.

Once you understand light bulb codes, things get a little simpler, so let’s explore:

• What A19 and A21 mean in detail
• Whether they’re interchangeable
• A19 and A21 base sizes

What Does A19 And A21 Mean?

There are a lot of different codes related to light bulb shapes and sizes, but you can split them into two camps.

One code will always refer to the bulb’s base – how it is connected to a light fixture.

These codes will generally start with an E, a B, or a G. E-type bulbs are Edison bulbs and have a screw fixture – these are the most common in the US.

B-type bulbs are bayonets with two pins that must be inserted and twisted. And G-type bulbs are spotlights with pins that push into place.

After the letter, there’ll be a number referring to the base diameter, or the distance between the pins, in millimeters.

Now that that’s out of the way, any other letter/number code you see on a bulb’s packaging or product description will describe its shape and size.

The most common are A and C, but there are all different kinds.

‘A’ bulbs are called arbitrary and are the typical rounded pear shape you associate with a classic light bulb.

‘C’ is candle-shaped, starting wider before tapering to a point.

The number in these codes represents the diameter of the bulb.

Bulb measurements are given in eighths of an inch, so an A19 bulb is a rounded pear bulb with a diameter of 19 eighths of an inch, or 2 and 3/8 inches.

That’s right – bulb base measurements are given in metric measurements, while the bulb size measurements are given in imperial measurements.

While the code will refer specifically to the bulb’s width, these bulbs also have a standard height.

So, here’s how an A19 and A21 bulb compare:

Bulb	Base	Diameter	Height
A19	Normally E26	2 3/8”	4.13”
A21	Normally E26	2 5/8”	5”

Are A19 And A21 Bulbs Interchangeable?

In theory, A19 and A21 bulbs are interchangeable because the codes don’t refer to the base.

Most of these bulbs have the same E26 base, so they can be swapped in or out at will as the base connection is identical.

The only problem comes with the shape and size of the rest of the fitting.

Any fitting designed for an A21 bulb will easily fit an A19 bulb because the A19 bulb is smaller in terms of diameter and height.

But if a fixture is specifically designed for an A19 bulb, or is just a smaller fixture in general, then an A21 bulb might be too wide or too tall.

It shouldn’t be an issue in pendant light fixtures, but any other bulb housing may just not have the space to accommodate the larger bulbs.

Even if you’re using LED bulbs that don’t generate anywhere near as much heat as older bulbs, you still need to ensure there is the air around all sides of the bulb to help with heat dissipation.

Having a large A21 bulb in a fixture touching the materials at the sides could be dangerous.

Generally, A21 bulbs will have a higher lumen output than an A19 bulb, though this isn’t always the case.

So, remember your choice of the bulb when replacing one and how bright you want the light to be – but always check the lumen output of the bulb you’re buying, and use that as a guideline more than just the size of the bulb.

A19 And A21 Bulb Base Sizes

As I’ve clarified, pretty much all A19 and A21 light bulbs use the same base size – an E26 screw fixture which is standard in a lot of fixtures.

In countries with higher line voltage, these may be the slightly larger E27. In the UK and some Commonwealth countries, the B22 bayonet fixture is another option.

Here’s a quick rundown of some of the more popular bulb shapes that you can buy, along with their base size (in both the US and in Europe)

Bulb Size	US base size	UK base size
A bulbs (A15-A23)	E26	E27, occasionally B22
C and CA bulbs	E12 or E26 (depending on size)	E14 or E27 (depending on size)
MR spotlights	GX5.3, GU4 or GU10	GX5.3, GU4 or GU10

Generally, if a light bulb is considered a ‘standard’ size, it will have an E26 base in the US.

E12 bases are reserved for smaller bulbs in candelabras or special small fixtures, and spotlights will usually have GU push-pin connectors.

Final Words

Most of the time, an A19 and A21 bulb will be interchangeable.

The difference in size between the two is negligible since we’re talking fractions of an inch, and the base sizes are identical.

I’d recommend checking the light fixture if you’re already using an A19 and ensuring there’s plenty of space around the bulb. Then, if you swap it for an A21, there won’t be any issues.

It’s not worth risking the bulb overheating and becoming a hazard.

Got any more questions about arbitrary bulbs or light bulb sizes in general?