Any guide to Bi-pin bulbs? (Bulb type, base type)


Apr 19, 2009
Cambridge, MA & Malaysia
Hi all,

I really don't know whether this has been discussed before, but I feel that information on this topic would have helped me and prevented me from making some costly mistakes a while back when trying to buy bulbs/sockets/reflectors.

I have done searches (both CPF google as well as the board search function), as well as looked at the threads of interest (and external links such as the 'great internet light bulb book', etc) but haven't found anything satisfactory.

I'm not sure if it is assumed that people know about this stuff if you're a regular light bulb user, but especially coming from the fluorescent world, I had no idea of this stuff.

Two of my biggest questions revolve around the bulb type and base types of bi-pins. From what I have gathered, there are two specifications:

T__, i.e. T1, T1.5, T2.25, T2.5, etc. and that this refers to the diameter of the bulb itself, in multiples of 1/8", so T1.5 would be 0.1875 inches. (From Carley's website on "Tech Speak")

G__, i.e. G4, G5, G6.35, etc. and that this refers to the distance between the pin legs in mm. so G4 would have pin separation of 4mm. (On Wikipedia's bi-pin entry)

And then there are length (well, easy enough) and filament specs...

Is this right? And are there any threads / resources on this? Sorry if there has already been a thread on this, but I haven't been able to find any...


Flashlight Enthusiast
Jun 28, 2007
Melbourne, Australia
You're right about the T- numbers being the bulb size in eighths of an inch.
The "T" stands for "tubular".

Just in case anybody is interested, other glass bulb envelope codes are as follows:
G: globular
S: "straight sided" (conical shape, fanning out from the base)
ST: a mix of straight-sided and tubular, very common in pre-war radio valves
A: the classic household light bulb shape

As for your socket dimensions, try here: