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.
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:
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