Edison Bulb Shape Code

If you've been on the hunt for Edison bulbs, chances are you've come across those shape code numbers in the bulb's description. You may have seen terms like ST18, ST58, ST21, ST64, G25, G30, G40, A19, A21, and A23. But what do these codes actually mean?

I wanted to address this topic because I've noticed that the shape codes for vintage light bulbs sold in the US are now presented in two different measurement systems, which can be quite confusing.

The two measurement systems in question are the imperial and metric systems. As you probably already know, the US uses the Imperial system, which measures distance in inches, feet, yards, and miles. On the other hand, most other countries use the metric system, which measures distance in millimeters, centimeters, meters, and kilometers.

Now, here's where the confusion sets in:

The most popular Edison bulbs, such as the ST18 and ST21, are labeled with codes in inches in the US. However, many online stores now list these bulbs as ST58 and ST64, which are the same bulbs but measured in metrics. So, for example, if you come across a fixture that requires ST58 bulbs, you now know that it's the same as an ST18 bulb.

To make things easier for you, I've put together a table that showcases some of the most popular vintage bulbs and provides both the imperial and metric codes for each bulb. This way, you can quickly reference both codes and avoid any confusion when shopping for Edison bulbs.

By understanding these shape codes and the measurement systems they correspond to, you can confidently navigate the world of vintage light bulbs and make informed decisions when purchasing the right bulbs for your fixtures.

Remember, whether you're looking for ST18, ST58, ST21, ST64, G25, G30, G40, A19, A21, or A23 bulbs, knowing the different codes and their corresponding measurements will help you find the perfect fit for your lighting needs. Happy bulb shopping!

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

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: