Bulbs: Their shapes, sizes, and bases

November 19, 2018

Today’s Question:   What cord do I need for a G40 bulb? C7? E17? What??

I got this desperate question from a friend with a stage lighting crisis.  I talked him down off the ledge and connected him with the lights he needed for his band’s upcoming performance.

(He was looking for a G40 cord what he needed was E12. Read on for more clarification.)

We also hear the question in these forms:

  • What are the differences in C7 and C9 bulbs?   
  • How big are G50 bulbs?
  • What cord do I need for my C9 bulbs?

And so on.

There is just so much “industry language” that can prove confusing as shoppers browse all the bulbs that are available.

Easy Answers:  The numbers in C7 and C9 are their diameters so that naturally means that C9 bulbs are larger than C7.  G50’s are round bulbs and the 50 is their diameter in millimeters which is 1 7/8 inches. Choose a cord based on the size of the threaded base on each bulb.  C7’s use E12 cords. C9’s use E17 cords.  G50’s might use either E12, E15 or E26 cords based on their specifications.

It’s easy once you know the lingo.

Please enjoy this bulb size and base tutorial.

This photo shows a sampling of bulbs in various sizes with one of three bases sizes. These are not all the bulbs that I sell but it’s a nice representation that provides relative size information and show the extent that different shapes, sizes and bases can be combined.

The first designation is the shape of the bulb, the second is the base.  For example, C9/E17 means that the bulb is a C9 shape with an E17 base.  Note, as a rule of thumb, traditionally C9 bulbs “always” have E17 bases so many times, the base is also referred to as C9 as well.




Basic Shapes for Christmas light and outdoor string light bulbs

C Shapes – cone

“C” stands for Cone – this is the almost pointy shape that is most traditionally associated with Christmas lights

Typical bulbs:  C7, C9 and C6 with C5 and C3 to a lesser extent.

The number after the C represents the diameter divided by 8:

C7 bulb: 7/8 = 7/8″ diameter


C9 bulb: 9/8 = 1-1/8″ diameter



See this blog that compares C7 and C9 bulbs and shows their dimensions side by side.

G Shape – Globe (round)

Typically seen in G30, G40 and G50 bulbs.  The number is the diameter in millimeters.  Since 50 mm is tricky to visualize keep this in mind:

G30 – slightly smaller than a ping pong ball
G40 – about the size of a ping pong ball
G50 – slightly larger than a ping pong ball



“T” represents elongated bulbs like a tube and is an industry identifier

Seen in T50 bulbs.  50 is the diameter of the bulb at it’s widest point.



“S” is for a shorter round patio bulb shape

Seen in S14 bulbs.



Base Sizes

There are 3 common threaded sizes of bases of bulbs in the US.  The numbers represent the width  of the base in millimeters.

Candelabra: E12, the size of a traditional nightlight

Intermediate: E17 — commonly seen on C9 bulbs

Medium or standard: E26, commonly seen in patio string lights and bulbs designed originally with a commercial market in mind but now seem more available for residential users as the market has seen increased demand for outdoor string lighting

And in Confusion…. Conclusion….

Traditionally C7 and C9 cords and bulbs are just called …  “C7 cords and bulbs and C9 cords and bulbs” or “C7 Christmas lights” or “C9 Christmas lights”.  So make sure you match them when you select the incandescent or LED bulb of your choice match it with the correct cord.

For example:   C7 bulb to C7 cord

If you are working with G, T or S style bulbs, look for the size of your base listed in the bulb’s specifications and match to the right cord.

For example:  your G40 bulb might have an E12 or an E17 base and you’ll need to find the appropriate E12 base cord (which will be called C7 more than likely)  or the right E17 cord (which will be called a C9 cord)

The combination of bulbs and bases is growing so just double check to make sure you have a match.

We’ve tried to make life easy at Christmas Light Source by listing a sample of the correct bulbs and cords for each of our products.  Hopefully this will help point you in the right direction.

Let us know if you have any questions. You’ve got this.


Shellie Gardner
Aside from throwing dinner parties that feature at least two kinds of cheese dip, Shellie's passions include travel, Mid-Century Modern furnishings and finding the perfect street taco. Has been known to snort laugh champagne.

12 thoughts on “Bulbs: Their shapes, sizes, and bases”

  1. Wow Shellie!
    I was just looking for the number associated with the Christmas trees bulbs that I have so I could order replacements. What I found were a whole bunch of sites trying to sell me some bulbs. Then I hopped into your blog. What a thorough delight. What I mean is, you have covered everything AND included phots and links.
    I can’t wait to delve more into your blog.
    Thank you.

  2. Please respond! What type of base do the miniature “Push In” Christmas light strings called? I have a hobby type light that uses this type of bulb and would like to find the same bulbs in 12 Volt DC LED’s. I would like “Bright White” 12Volt push-in LED’s to replace the incandescent stock bulbs.
    Thank You!

    1. Hello William,

      Unfortunately, the electrical socket/specifications for incandescent mini light bulbs are not compatible with any type of LED replacement bulbs. There is not a direct incandescent to LED replacement bulb available at this time due to the significant differences in the way they produce light.

      You will have to replace the entire assembly in your model to go to LED.

  3. Great article, but it didn’t address my string of Christmas lights which have threaded plastic ‘bulbs’ in the traditional shape of C9. However, when I unscrew the bulb, I then have small LED lights that pull straight out. Their base looks like the old “mini lights” with the two wires that push through the holder and bend backwards before sliding into the lamp socket. I’m not sure of the correct terminology, but my string has a couple burnt out LED lights and I need replacements. Do you have these? Are they inferior to the LED bulbs with the traditional threaded C9 base?

    Perhaps Home Depot saw me coming!!!

    1. Hi Rick,

      Those sound like custom sets that use a proprietary bulb and socket configuration. Contacting the manufacturer is your next best step to track down replacement bulbs since standard C9 retrofit bulbs won’t be compatible with your strings.

      If you can’t reach the manufacturer, consider using one strand as a “donor” for the others as they need replacement bulbs.

      I can’t speak to the issue of superiority but traditional threaded C9 bulbs that fit into standard cords are a lot more convenient.

      Hope this helps! (Sorry that we had a little glitch here on the blog and am just responding to you.)

  4. Hello there. In the picture of the “G” bulbs, it shows the G40 as being 1 3/8″ in diameter, however, it is actually about 1 5/8″ diameter. Just a little over 1 1/2 inches.
    Also, that first picture with all the bulbs in an arch, the pink faceted G50 E12 is actually at G40.
    Please make a note of it 🙂

    1. James,

      Thanks for your comment!!!

      For fun, I just put an LED G40 bulb into a spanner clamp and it measured 1 and 9/16 inches! So, yep, just that tiny smidge over 1.5!

      Since 1.5 inches will mean more to most folks, we’ll update that in the image as well as make that G40 to G50 correction in the header graphic.

      It’s a great time to make that image look more updated, too!

      Thanks again,


Leave a Reply

Your email address will not be published. Required fields are marked *

The maximum upload file size: 5 MB.
You can upload: image, audio, video, document, spreadsheet, interactive, text, archive, code, other.
Links to YouTube, Facebook, Twitter and other services inserted in the comment text will be automatically embedded.

You May Also Like...

DIY: Milk Carton Ghosts
DIY: Milk Carton Ghosts
June 20, 2019

We love a fun project for Halloween! Our official first spooky project for 2019 is Milk Carton (Jug) Ghost, Luminaries. We’ve been saving the milk jugs for this project for a while and we’re happy Read More