Where keg sizes actually come from
A "keg" isn't one fixed size; it's a fraction of a barrel, and the barrel itself is a legal unit of measure. The US Alcohol and Tobacco Tax and Trade Bureau defines a barrel as 31 gallons for beer-tax purposes (27 CFR 25.11), and separately authorizes the fractional and metric container sizes breweries actually fill (27 CFR 25.156): halves, thirds, quarters, sixths, and eighths of that 31-gallon barrel, plus straight 5-gallon kegs and metric 30 L and 50 L kegs. The everyday keg names come straight out of those fractions: a half barrel (the size most people mean by "a keg") is 15.5 gallons, a quarter barrel or pony keg is 7.75 gallons, and a sixth barrel or sixtel is 31/6, about 5.17 gallons.
Two sizes on this calculator sit outside that legal fraction system. A "corny" or Cornelius keg is a 5-gallon container that TTB separately authorizes as a keg size, but the name comes from its original use as a soda-fountain syrup keg that homebrewers repurposed; it isn't a clean fraction of 31. A mini or party keg, the small stainless container sold at grocery and liquor stores for a single gathering, typically holds 5 liters, the metric size several import brands use for retail party kegs.
Two worked examples
A half barrel filled with a standard 12 oz can-equivalent pour: 15.5 gal converts to 58,673.9 mL (using the exact US-gallon-to-milliliter factor), and a 12 oz serving is 354.9 mL, so 58,673.9 / 354.9 = 165.3, which rounds down to 165 full servings. Switch the same half barrel to 16 oz US pint pours instead, and the math comes out exact: a half barrel is 1,984 US fluid ounces, and 1,984 / 16 = 124 pints, no rounding needed.
A sixth barrel (sixtel), the size a lot of taprooms use for a rotating tap, holds 31/6 gallons, which is 19,558 mL. At a 12 oz serving that's 55.1, so 55 full pours, matching the same figure published by keg-size guides like Kegerator's Learning Center. Swap in a 20 oz UK imperial pint instead and the count drops to 34, because a UK pint pour holds more liquid than a US pint, not because the keg got smaller.
US pint vs. UK pint, and how much to budget for waste
A "pint" isn't a single measurement worldwide. A US pint pour is 16 US fluid ounces (473.2 mL); a UK or Irish imperial pint is 20 imperial fluid ounces (568.3 mL). The imperial fluid ounce is actually the smaller of the two units, about 3.9% less than a US fluid ounce, but a UK pint counts 20 of them against the US pint's 16, a 25% bigger count that more than makes up the difference. Net result: an imperial pint holds about 20% more liquid than a US pint. Pick the wrong one and a keg-servings estimate is off by a fifth before anything else changes.
None of the numbers above account for foam and spillage, which is why this calculator has a separate waste-allowance field defaulted to zero rather than baking in an assumed loss. For commercial draft systems, draft-technology vendor BevTeq puts typical bar and restaurant draft-beer waste in the 15-25% range, mostly from foam; the Siebel Institute of Technology's draft-loss guidance points the same direction (a pour that's mostly foam wastes most of its liquid beer) without naming a percentage of its own. Well-maintained systems do better than that range. A home kegerator or party pump, poured by hand into varied glassware, doesn't have a published equivalent figure, so treat that industry range as context for setting your own waste allowance rather than a number this calculator assumes for you.
Linking keg volume to everyday liquid units
Keg sizes here are expressed in US gallons and liters, but party planning questions often arrive in cups or fluid ounces ("how many 12 oz beers"). The volume converter translates those kitchen and bar units using the same exact US-gallon definition this calculator uses for keg fractions, so you can double-check a custom pour size before dividing keg volume by it.
Food quantities for the same event run on a different formula entirely: the pizza calculator estimates how many pies to order from guest count and slices per person rather than from draft-beer geometry.