It’s not something I lose sleep over, seeing as my scales aren’t pinpoint accurate anyway, but I’ve noticed in my homebrew reading that there isn’t a suitable calculation for getting IBUs or ounce weights for British gallons. Papazian has a nice one for American homebrewers using grams, and even goes as far as to suggest an equivalent for British measures, but in spite of the use of ounces, which weigh the same in the US and the UK, the equation still figures in US gallons, which, as I’ve already said somewhere, is a bit of a wussy gallon. So, here is an adapted equation I’ve been working on for a couple of hours.
Firstly, you can either use the original Papazian sum, which is: and then correct it for UK gallons by dividing the answer by 19 (the number of litres in 5 US gallons - 5 gallons being a typical batch size) and multiplying that by 22.5 (the number of litres in 5 UK gallons), or by multiplying the answer by 1.1842. Those will give you an answer in ounces, and they shouldn’t be miles out.
Or you could use my own simpler contribution to Imperial homebrewing, by using the “Evans UK factor”, or the EKs Factor!
The Eks Factor
The Eks Factor is a simple number that you can use within the above equation that will give you an answer for your common or garden UK gallon brew. So instead of 1.34, use the number 1.587, and thus the equation becomes:
So, for a 5 UK gallon brew, with an IBU of 50, hop utilisation of 30%, an Alpha Acid content of 6% and the Eks Factor, 1.587, the sum goes as follows:
To get an accurate answer you do need to use the factor to 3 decimal places, but then, if like me your scales don’t go much lower than an ounce, I’d recommend, in Papazian’s own words, “don’t worry, have a homebrew”.
I’ve looked through about half of the books on homebrewing available, and the older UK books tend to only use AAU and HBUs in the calculations, which is too general for me. The American books, for example Papazian’s The Complete Joy... doesn’t account for the larger UK gallon, and the exquisitely detailed Designing Great Beers by Ray Daniels has something that’s far too complicated for me to get a straight answer out of – my head just can’t get round it! Brew Classic European Beers At Home has an equation in grams that can be converted into ounces if needed, by dividing the gram answer by 28.35 (the number of grams in an ounce), although that answer will be about 0.002 oz out!
Anyway, to sum up, for a very easy equation for anyone brewing in macho-gallons, use the following: I’m no mathematician, and given that I’m not even sure my hop utilisation IS 30%, it really is just splitting hairs. In order to accurately assess your hop needs or IBUs, colour of your beer etc. read Ray Daniels’ Designing Great Beers. But if you’re merely staggering drunkenly towards your perfect pint, my Eks Factor will suit you nicely! Good luck, and happy nit-picking.