A21 Light Bulb: Dimension & Compatibility with Lamp Holders

When you buy an A21 LED lamp, it states that it is an A-Type LED lamp. It should be able to meet the requirements of an A21 lamp. You can bet that if you bought a glass incandescent light bulb labeled A21, the bulb would measure A21. In this article, we’ll learn everything there is to know about the A21 light bulb.

“A” Bulb Shape Family

The A-shape bulb was the first widely used household bulb, and it is what most people think of when they hear the word “light bulb.” For living room lamps, closets, vanities, ceiling fans, multiple-bulb ceiling fixtures, and covered porch lights, the “A” bulb form remains the most popular.

What does the term A21 mean?

What does the term A21 mean
What does the term A21 mean

The word A21 describes a light bulb’s overall shape and proportions. A21 light bulbs are less well-known than their more famous counterpart, the A19 bulb, and are more typically employed for more robust light output applications. A21 comprises two parts: the first letter and a two-digit number. The two digits following the letter denote the bulb’s diameter at its widest point, which is measured in eighths of an inch. The diameter of an A21 bulb is 21 divided by 8 inches or approximately 2.6 inches. Compared to an A19 bulb, which has a diameter of 2.4 inches, this is a significant difference.

***Read more: E26 vs E27 Bulb: Mechanical and Electrical Safety Differences

Dimensions of an A21 light bulb

An A21 bulb has a diameter of around 2.6 inches by definition. A21 bulbs are also known as A67 bulbs in countries that use the metric system, where 67 refers to the diameter in millimeters.

A21 bulbs are slightly longer than A19 bulbs, ranging from 4.1 to 5.4 inches (105-140 mm) in length.

Dimensions of an A21 bulb
Dimensions of an A21 bulb

The ANSI C79.1 rules govern the standard size of an A21 bulb in North America. Varying bulb sizes have different maximum diameters for different wattages:

– 100W (Version 215):

112.7 mm max length

75.5 mm max diameter

– 100W (Version 216):

134.9 mm max length

78.0 mm max diameter

– 150W (Version 262):

139.7 mm max length

78.5 mm max diameter

If an A21 bulb is within 139.7 mm in length and 78.5 mm in diameter, it is generally considered within its dimensional standards.

A21 incandescent lamps are A19 incandescent lamps with a higher wattage. The more oversized bulb shape must properly contain and dissipate the heat created due to the increased wattage.

***Read more: The Difference Between B11 vs E12 – Where Are B11 E12 Bulbs Used?

A21 light bulbs always use E26, but not vice versa

A21 bulbs in E26 base
A21 bulbs in E26 base

A21 bulbs must permanently be mounted in a light fixture with an E26 base, according to ANSI requirements.

Although the terms A21 and E26 refer to various things, in this situation, all A21 bulbs use E26 lamp bases.

However, remember that the opposite is not valid: not all E26 bulbs are A21-shaped. Some E26 bulbs have radically different forms from a conventional A21 or A-style bulb.

What about A21 CFL and A21 LED bulbs?

Because of these lights’ more significant market possibility, most energy-efficient bulbs are designed around the A19 form factor.

On the other hand, LED lamps are far more heat-sensitive than incandescent bulbs. Because a larger bulb body allows for faster heat dissipation into its surroundings, a larger LED bulb can handle more brightness.

As a result, some brighter A-style LED bulbs (e.g., 100W equivalent) will have a more significant form factor than A19 and fall into the A21 category.

_A21 LED light bulbs
_A21 LED light bulbs

***Read more: Top 6 Best LED Filament Bulbs: Perfect Choice for Classic Style

Are A19 lamps compatible with A21 lamp holders?

A19 lamps will fit in practically all fixtures and lamp holders designed for A21 lights since they are smaller in all dimensions than A21 lamps. The A19 lamps will fit into the socket perfectly because they share the same E26 base.

Are A21 lamps compatible with A19 lamp holders?

This one is a little more difficult because certain A19 lamp holders aren’t made to handle a larger bulb.

A19 lamp holders are also unlikely to be rated for use with higher-wattage bulbs like A21 incandescent lights.

The reason for this is twofold: first, the wiring and socket may not be rated for higher current demand, and second, the lamp fixture may not be designed to handle the extra heat produced. Second, a larger A21 light bulb may cause the heat to be concentrated closer to the lampshade or other apparatus, posing a fire hazard.


After showing up, we can safely stick and base our verdict on features and uses, as indicated in the review. And, as you may be aware, the A21 light bulb is a better choice for your home. It has a bigger diameter, holding the appropriate quantity of energy/heat from the power. You should also be aware of the significance of differentiating these goods. However, if you list the bulb’s advantages and disadvantages, it will be helpful. However, if you’re interested in the numbers, you should go back and look at these minor details.