August 04, 2019 2 min read 3 Comments

If you been shopping for Edison bulbs you most probably have seen those shape code numbers in the bulb's description. For example ST18, ST58, ST21, ST64, G25, G30, G40, A19, A21, and A23. What do those codes mean?

The reason I decided to write about this now is that I've noticed the shape codes of the vintage light bulbs being sold in the US are now presented in two different measurement systems which can be very confusing.

The two measurement systems are the imperial and metric system. As you already know here in the US we use the Imperial system. We measure distance by units of inches, feet, yards, and miles. Most other countries use the metric system which measures distance by units of millimeters, centimeters, meters, and kilometers.

Now here is the confusing part: 

The most popular Edison bulbs are the ST18 and ST21. Those are the codes in the US (inches). Many online stores label those bulbs as ST58 and ST64 which is the same thing but in metrics. So for example, if you buy a fixture and it calls for ST58 bulbs, now you know it the same as ST18.

I put together this table for some of the most popular vintage bulbs so you can easily see both codes for each bulb.

US System Metric System
ST18 ST58
ST21 ST64
G25 G80
G30 G95
G40 G125
A19 A60
A21 A67
A23 A70
T14 T45


So what do those letters and numbers mean?

1. The numbers represent the bulb's diameter. You will need to multiply the number by 1/8 to get the diameter in inches. So, the ST18 is 18 times 0.125 which is 2.25. This means that the diameter of the Edison ST18 bulb is 2.25". The same applies to the rest of the bulb codes.

2. The letters represent the shape of the bulb.  See chart below of the different bulb shapes:

Bulb Shape Code Reference Chart


3 Responses

Christmas lights nz
Christmas lights nz

August 18, 2021

I loved your blog and thanks for publishing this content about Edison bulb shape code!! I am really happy to come across this exceptionally well written content. I like how you have researched and presented these exact points so clearly. Thanks for sharing and look for more in future!!


August 16, 2020

Thank you for explaining this. You made my day!


August 16, 2020

Nice Post! Edison bulbs are the most popular shape of antique bulbs. Edison bulbs have an oblong shape with a nipple at the top and typically have a medium E26 base.

Leave a comment

Comments will be approved before showing up.