As with many traditional systems of measurement definitions of common terms such as barrel vary from country to country. When it comes to casks, British terminology seems to rule. The most common size of cask is a Firkin at 9 imperial gallons or ¼ barrel. The standard keg size at The Beer Store follows American traditions and is ½ barrel or 15 ½ US gallons (12.91 imperial gallons or 58.67 L).
Cask sizes: Pin (4.5 gal.), Firkin (9.0 gal.), Kilderkin (18 gal.), Barrel (36 gal.), Hogshead (54 gal.), Butt (108 gal.), Tun (216 gal.).