Published on September 13th, 2014 | by Peter Young


A Guide To Understanding Modern Light Bulbs: Base Types

light bulb base article

As if there wasn’t enough to consider when it comes to picking out the right light bulb, being sure that you have the correct base type and size is yet another factor in your decision. Thankfully, this is probably the most straightforward of all the variables when it comes to purchasing a light bulb. To help you understand the four most common base types, we’ve come up with the following guide.

Modern Light Bulbs: Base Types

These are the four most common light bulb bases that you’ll encounter around your home:

  • Medium Screw Base (E26 are common in the U.S., E27 in Europe. E is for “Edison”, by the way)
  • Candelabra
  • GU
  • Bi-Pin

To help you get a better idea of what these base types look like, take a look at the following image:

bulb bases

From left to right, Medium Screw Base, Candelabra, GU and Bi-Pin.


As you may have guessed, some types of light bulb shapes will often be paired with a certain type of light bulb base. This is because some styles and shapes of light bulb are designed with a specific use in mind, which will require that a certain type of light bulb base be used in order for the light bulb to work properly. For example:

It’s quite common for track lighting to feature halogen style light bulbs, which come in an MR shape. In order for these bulbs to fit into the socket of the fixture, they will have either a GU or Bi-Pin style of base and will look something like this:


You’ll also find that light bulb bases will come in a variety of sizes, which is determined by measuring the width of the light bulb base in millimeters at its widest point. All base types, regardless of style, are measured in this way. The size of the base is expressed as number and will appear right next to the base style on the packaging. For example, a light bulb with a GU-10 base is 10 millimeters wide.

Take a look at the following image to get a better idea of how this measurement is taken:


It’s also important to note that any given shape of light bulb can come with a variety of different base types. Check out the following image to get a better idea of what we’re referring to:


The base types (from left to right): GU10, GU5.3 and GU4 or GZ4


If you’re looking for more ways to make your home energy efficient, try checking out some of our green home improvement projects: Green Living Ideas, after all, is a top 20 home improvement website!

Photos courtesy of Phillips, LowesLightBulbs, and Wikipedia.

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About the Author

graduated from Pacific Lutheran University (PLU) with a degree in journalism and has made sustainability and eco-conscious living mainstays of both his professional and personal life. It was during his time at PLU that he began his journey with sustainability and it's what has led him to writing for Green Living Ideas. He currently resides in Honolulu and works for Pono Home, an energy efficiency company focused on reducing carbon emissions and promoting a healthier, greener lifestyle.

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