How To Read Light Bulb Ordering Codes

reading light bulb codes

Let's begin the lighting eduction with a section for my friends who lack the enthusiasm to learn about the world of light bulbs.

Step 1 to understanding Light Bulb codes is to Disregard Step 1

Snap a photo of the light bulb you currently use, take one picture where you zoom in on the bulb markings, note the bulb location (be it the kitchen, bathroom, restaurant, or warehouse), figure out the quantity of bulbs required, specify your preferred light type or hue, and forward all these details to a lighting expert at GoodBulb. Request assistance from the Lumen Master, our guide poised to help. Lighting is our forte, and understanding codes, and replacements is our specialty.


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Light bulb ordering and description codes may seem like a random assortment of letters and numbers, but they're actually a standardized way to convey a bulb's specifications quickly and accurately. Understanding these codes can help you choose the right bulb. Here's a breakdown of how to read these codes:

Base Type
The base type of a bulb is usually indicated by one or two letters at the beginning of the code. For example: E stands for Edison screw base common in many bulbs. The number following this letter indicates the diameter of the base in millimeters. 

  • E26 means an Edison base 26 millimeters in diameter, which is standard in the U.S.
  • B indicates a bayonet base, less common in the U.S. but found in many applications.
  • GU is used for bi-pin twist lock base. The number following GU indicates the distance between the pins in millimeters.

Bulb Shape
The next part of the code refers to the shape of the bulb, and it's often represented by one or two letters:

  • A stands for Arbitrary (standard shape), which is the classic light bulb shape.
  • BR indicates Bulged Reflector, commonly used in flood lights.
  • G refers to Globe, a spherical bulb often used in vanity lighting.
  • T stands for Tubular, with a long, narrow shape.
  • PAR stands for Parabolic Aluminum Reflector
  • HID stands for High Intensity Discharge
  • X stands for Xenon
  • XL stand for Xtreme Life

Bulb Size
After the shape designation, a number often follows, which refers to the bulb's diameter at its widest point, divided by 8. So, an A19 bulb is an Arbitrary shape bulb that is 19 eighths of an inch (or 2.375 inches) in diameter.

Additional letters or numbers at the end of the code might indicate special features or the technology used in the bulb:

  • D can indicate color temperature, Daylight 5000-6500K
  • W indicates wattage, the amount of energy the bulb uses.
  • L or LED specifies that the bulb uses LED technology.
  • H could indicate that the bulb is halogen.
  • D could also mean the bulb is dimmable.

Examples of Light Bulb Codes

  • E26 A19 code means the bulb has an Edison 26mm screw base and is in the classic Arbitrary shape with a diameter of 2.375 inches.
  • GU10 BR20 indicates a bi-pin base 10mm apart, with a Bulged Reflector shape and a diameter of 2.5 inches.
  • T8 code signifies a Tubular bulb 1 inch in diameter (since 8 divided by 8 equals 1)

By breaking down the code piece by piece, you can determine the base type, shape, size, wattage, features, and kelvin temperature. This knowledge makes it easier to find the right bulb for your fixture, ensuring compatibility and meeting your lighting needs.


Every lighting technology has used a standardized set of codes that differentiate 10s of thousands of light bulbs. Buying LEDs is hard and I recommend working with a GoodBulb lighting specialist to prevent unnecessary returns. 


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