A standard keg of Guinness holds 124 pints. As a pint is 16 ounces, each keg contains 1,984 ounces of beer.
However, that number only accounts for the number of ounces in the keg. When a beer is drawn, it usually has a layer of foam, known as the head, which takes up more space than beer. Therefore, when taking into account the foam, a standard keg could serve as many as 140 pints of beer, if each glass has about a 2-inch head of foam. A standard keg is defined as half a barrel, so a barrel of beer would serve 248 pints.