There are so many types of light bulbs available on the market that it’s hard to keep track of them all, but each type has its own “code” of sorts that tells you everything you need to to know about it.

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Whether you’ve shopped around for smart light bulbs or just regular bulbs, you’ve likely come across bulbs that have something like “A19” or “E26” in the name. This isn’t the model number of the bulb, but rather the type of bulb, which is a universal standard coding system in the lighting industry.

A bulb is categorized by three different factors: shape, size, and base. These three factors are all defined in the bulb’s code, but that code is pretty useless if you aren’t able to decode it in the first place. Luckily, we’re here to help.

Defining a Bulb’s Shape and Base

As you might already know, light bulbs come in all kinds of different shapes. The single letter in the bulb’s code represents those shapes.

One of the most common shapes that you probably associate with a light bulb is the “A” series bulb with an “E” series base, “A” standing for Arbitrary and “E” meaning Edison.

Another common bulb shape is the B series bulb, which stands for Blunt tip, and are used in decorative light fixtures when you want a candle-like appearance. C and CA series bulbs are also similar, meaning Candle and Candle Angular, respectively. They still use the same E-series base, albeit with a smaller diameter.

Then there are G series (Globe) bulbs, which are entirely spherical.

You’ll also find a lot of BR series (Bulging Reflector) bulbs in recessed can light fixtures, and PAR (Parabolic Aluminized Reflector) series bulbs in outdoor flood light fixtures. Both have similar shapes but deliver vastly different types of light.

There are also different types of bases other than just the E series screw-on base. For instance, the G series (not to be confused with the G series bulb itself) base uses two pins instead of a screw-on style for low-voltage light fixtures (track lighting, etc.).

Of course, there are a lot more types of bulbs than the ones I mentioned, but Bulbs.com has a fantastic graphic that shows you all the different types of bulbs and their letter codes, as well as one for all the different types of bases.

What the Numbers Mean

So now that you know what the letters mean, the numbers can be just as confusing. Basically, though, the number corresponds to the bulb’s diameter at its widest point. The way it’s measured, however, is different for the bulb’s size and its base size.

A bulb’s size is measured in 1/8-inch increments, so an A19 bulb would be 2-3/8″ in diameter because there are nineteen 1/8-inch increments. An easier way to determine the diameter, though, is to take the number and divide it by 8. So 19 divided by 8 is 2.375, or 2-3/8 as a fraction.

The bulb’s base is measured differently but in a much easier fashion. The number refers to the diameter of the base in millimeters. So a bulb with an E26 base is 26 millimeters.

Or if we’re talking about the G series two-pin base, it would be the distance between the two pins measured in millimeters.

How to Use These Codes When Replacing Bulbs

More often than not, you probably don’t need to know the code of the bulb when finding a replacement. A majority of light fixtures use the E26 base, and pretty much every lamp and light fixture in your house uses an A19 bulb.

However, if you have a unique light fixture that accepts a different kind of light bulb and aren’t sure of the exact type you need to replace it, there are a couple of things you can do.

First off, it’s rare when the code is printed on the bulb itself. Usually, you can find it on the bulb’s packaging when you buy it, but it’s likely that you don’t keep the packaging after you buy a new bulb.

If you don’t know the code to a bulb that you need to replace, it’s not a big problem. You always can take the old bulb to the store and compare it to other bulbs to find a match. Light bulb sizing standards are different enough that it’s easy to spot the right size just by loooking at them. It’s certainly not like screws and bolts where being a single millimeter off can make all the difference.

Craig Lloyd Craig Lloyd
Craig Lloyd is a smarthome expert with nearly ten years of professional writing experience. His work has been published by iFixit, Lifehacker, Digital Trends, Slashgear, and GottaBeMobile.
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