August 04, 2019 2 min read

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). Now the same shape and sized bulbs are also marked 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


Leave a comment

Comments will be approved before showing up.

Also in Vintage Bulbs Blog

Edison Bulbs Brightness Questions
Incandescent Edison Bulbs - Two most asked questions:

June 30, 2019 2 min read

As we approach 8 years of selling vintage Edison bulbs I can tell you the two most important and most asked questions about Edison Bulbs.  These important facts are also the main reason customers return them or unhappy with them.  

What you must know before buying Incandescent Edison Light Bulbs:

Read More
Antique Hanging Bulbs
Are Edison Bulbs LED Bulbs?

February 04, 2019 2 min read 1 Comment

Edison light bulbs are reproduction bulbs which are made to look like the first light bulbs introduced by Thomas Edison over 100 years ago. They are incandescent bulbs with filaments and lens (Glass) which are shaped like the original Edison bulbs.
Read More
Vintage Swag Lamp
what are swag pendant lamps?

November 13, 2018 2 min read

Are Vintage Swag Pendant Lamps a Bright Idea for Your Home? 

Popularized in the mid-20th century, swag pendant lamps are making a resurgence in American home design, offering an affordable lighting option

Read More

News & Updates