Butt size, and other cask math

Let's do some cask math.

We'll start with the difference between a US gallon and an Imperial gallon (UK).

• A US gallon holds 128 US ounces, or 3.785 liters.
• An Imperial gallon, on the other hand, holds 153.7 US ounces, or 4.546 liters.
  
• So, one Imperial gallon is the equivalent of 1.2 US gallons.

Now, on to cask volumes.

• A firkin is a cask that contains 9 Imperial gallons. That's 10.8 US gallons, or roughly 40.9 liters, or 86 US pints (even though the last few would be quite 'sludgy').
  
• A kilderkin is a cask that contains 18 Imperial gallons, or twice the volume of a firkin. That's 21.6 US gallons.
• An English barrel contains 36 Imperial gallons, or four times the volume of a firkin. That's 43.2 US gallons. (A US barrel contains 31 US gallons.)
• A UK hogshead would be 54 Imperial gallons. That's 64.8 US gallons.

And then, at 108 Imperial gallons (that's 129.6 US gallons), there's the ... butt.

At this point, I turned to British beer blogger and author Martyn Cornell, aka Zythophile, for assistance.

The US gallon is based on the old British "wine gallon" of 231 cubic inches, against the Imperial gallon, which is 277 cubic inches.

Wine came in pipes of 126 wine gallons, equal to 105 Imperial gallons, and a hogshead of wine was thus 63 wine gallons - halve that again, and round it down, and you end up with the US barrel, 31 (US or "wine") gallons.

So a 126-(US) gallon "butt", 104 (or 105) Imperial gallons, is really a pipe.

Beer, on the other hand, in the UK came in butts of 108 (Imperial) gallons, which breaks down into two (54 Imperial gallon) hogsheads and three (36 Imperial gallon) barrels.

So that's why a US hogshead is two US barrels, but a UK hogshead is one and a half UK barrels (or three kilderkins),

and why "pipe" and "butt" can be used as synonyms (being both equal to two US hogsheads, 126 US gallons) in the US, but not in the UK, where a pipe is three (Imperial gallons) smaller than a (108-Imperial gallon UK) butt

There'll be a quiz tomorrow. It's open blog, okay?
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• Cask volume measurements: here.
  
• Cask exterior dimensions: here.
  
• What's a CE? and other beer volumes: here.
  
• Bourbon barrel volume measurements: here.
  
• Wine volumetrics: here.