The casks of whisky define much of its flavour. While the time that the whisky matures inside a cask is critical, the type and the size of the cask are also important. The size is usually difficult to define since no ISO standard defines the volume of a standard cask.
In the US, the term barrel is usually used to refer to the wooden container that holds whisky, but, typically, this refers to regular casks that contain around 53 U.S gallons or 200 litres of whisky. This generally is the most used barrel size amongst American whisky producers. A barrel of this capacity is approximately 2 inches by 36 inches and the circumference around 26 inches. As noted, this is the regular size, so it is good to know the type before determining how much to pay for a barrel of whisky.
Casks even of the seemingly same size can vary in capacity, after all, they are handmade and are often dismantled and reassembled. Another notable issue when it comes to volume is the unit of measurement. Take, for instance, a Butt. A regular one comes in size of 500 litres, but there is a measurement called a butt, which is around 122 US gallons, typically 477 litres. Hogshead is one of the traditional casks used in maturing Scotch whisky. They are made from American White Oak (Quercus Alba) and are approximately 245 litres/ 65 US gallons / 54 Imperial Gallons. Their large size makes them better suited for a more extended period of whisky maturation. Butts are traditionally made from Spanish Oak, but there are many of them made from American White Oak.
Other large size casks include the Port Pipe, Puncheon, and Madeira Drum. These are approximately 400 litres or 109 Imp. Gallons. Medium size casks are around 200-400 litres or 544-106 US Gallons and include Cognac cask, Bordeaux, ASB, Barrique cask, and any Hogshead. Quarter casks are the smallest; they are approximately 45-50 litres. They have a higher ratio of wood-to-liquid when compared to standard casks and therefore tends to accelerate the maturation process.
With around 53 gallons of liquid on every barrel, you should be able to get around 150×750 ml bottles of whisky because of the loss of the angel share. The angel share is basically what is lost during maturation. A bourbon barrel with fresh spirit at 63% may yield around 175 litres after around 12 years of maturation. The ABV will have dropped by then but will be about 55-60%. Dilution to around 40-43% before bottling means that you can have around 200 or 250 bottles of whisky. Remember, other factors are at play here one of which is the size of the cask, and the Angel’s Share. Some casks can even drop below 40% ABV which is the legal minimum. If this is the case, then a cask has to be blended with a stronger whisky to bring it back above 40%. Such would not be a single cask whisky anymore, but it is worth considering when purchasing a cask.