for the colour, its- EBC x 1.7/volume
for the IBU it
should be IBU x 1.7/volume
however.....there have been big long discussions in other threads about the coopers IBU ratings....on their website, they give the above formula, but advise that it will come in 10-30% under that, due to yeast dropping bitterness out (which does happen, but not to that kind of degree). General consensus (with which I fully agree) is that a more
reasonable representation is just IBU/volume.....
example, the Original lager. IBU 390. in 23L, going off the (mathematically correct) formula, you get 390*1.7/23 = 28.8 IBU. This means that for a beer that is at 0.5 BU:GU (ie neutrally balanced, niether sweet not bitter), you could make it up to 1057 with malt....well over a kilo. If you made it with 1kg of malt, this would give ~ 1040, meaning that it would be 0.72 bugu - which is perfect for a pilsner. But anyone who has ever made this tin knows full well that if you add more than, say, 300g of malt, you get a very sweet beer indeed....so the numbers don't match.
Using the same example, but just IBU/vol, you get 390/23 = 16.9 IBU, which sounds much more reasonable...this means that malt up to 1034 would give 0.5 bugu. 1kg of malt giving ~1040 would mean that it is 0.42bugu - very sweet. If anything, I think 16.9 is generous for this kit.....
Having done many coopers kits, I have found that for volumes of 15-25 L, the straight division is reasonable for everything in the lighter colour range. Once you get to the dark and the stout, the bitterness appears to be closer to what coopers states, however I never really played with the darker kits, so can't be sure. But have done lots of Coopers draught, pale ale, sparkling, real ale and lager tins before moving away from kits, and always got good results with IBU/volume. (and we all know how I hate using formulae that are incorrect.
)....comparing the later batches to the full extract batches that followed, where I controlled the hopping, it works out as fairly close doing it this way.
Slightly OT, but hope this helps.