>I'm sorry to disagree with some others here, but flash guide >numbers do not have units - it is a just a number.
I’ll be the first to admit; I’m not the sharpest tool in the shed and have had no formal training when it comes to photography. Being self-taught for the most part I realize that I may have numerous misconceptions (although I don’t think this is one of them, but I could be wrong!).
So with that said, perhaps I’m misunderstanding this phrase which is an excerpt from this article over at Nikon’s imaging web site.
“Quote” “The flash guide number (GN) is a measure of the distance at which the flash can illuminate a subject.” “Unquote”
I take that to mean what I defined in my first response and that is the GN listed in the specifications is the maximum distance a flash can reach and assumes an f-number of 1 and an ISO of 100 unless otherwise stipulated.
After all, to be a measure of distance would imply that it is associated with a particular metric otherwise it would be a meaningless number. For example if I tell you I have two flashes, one with a guide number of 40 and another with a guide number of 50, which is more powerful? You then might tell me the one with the GN of 50 at which time I tell you “oh yeah I forgot, the one with a GN of 40 is in meters and the one with 50 is in feet” you would then say “oh in that case the one with a GN of 40 is more powerful”.
So in that regard I believe my first response directly answered the OP, albeit perhaps not clear enough. Of course that is unless I am under a misconception in which case I must ask, “what am I missing here?”