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.