Guinness ( ) is a popular dry stout that originated in Arthur Guinness' brewery at St. James's Gate in Dublin, Ireland. The beer is based on the porter style that originated in London in the early 18th century. It is one of the most successful beer brands in the world, being exported worldwide. The distinctive feature in the flavour is the roasted barley which remains unfermented. For many years a portion of the beer was aged to give a sharp lactic flavour, but Guinness has refused to confirm if this still occurs. The thick creamy head is the result of the beer being mixed with nitrogen when being served. It is extremely popular with the Irish and, in spite of a decline in consumption over recent years, is the best-selling alcoholic drink of all time in Ireland, where Guinness & Co. makes almost €2 billion annually.

The parent company has been headquartered in London since 1932 and was later merged with Grand Metropolitan plc and developed into a multi-national alcohol conglomerate named Diageo.

Composition

Guinness stout is made from water, barley malt, barley, hops, and brewer's yeast. A portion of the barley is flaked (i.e. steamed and rolled) and roasted to give Guinness its dark colour and characteristic taste. It is pasteurised and filtered. Despite its reputation as a "meal in a glass", Guinness only contains 198 kcal (838 kilojoules) per imperial pint (20 fl oz UK) (1460 kJ/l), fewer than an equal-sized serving of skimmed milk or orange juice and most other non-light beers.

Draught Guinness and its canned counterpart contain nitrogen (N₂) as well as carbon dioxide. Nitrogen is less soluble than carbon dioxide, which allows the beer to be put under high pressure without making it fizzy. The high pressure of dissolved gas is required to enable very small bubbles to be formed by forcing the draught beer through fine holes in a plate in the tap, which causes the characteristic "surge" (the widget in cans and bottles achieves the same effect). The perceived smoothness of draught Guinness is due to its low level of carbon dioxide and the creaminess of the head caused by the very fine bubbles that arise from the use of nitrogen and the dispensing method described above. "Original Extra Stout" contains only carbon dioxide, causing a more acidic taste.

Contemporary Guinness Draught and Extra Stout are weaker than they were in the 19th century, when they had an original gravity of over 1.070. Foreign Extra Stout and Special Export Stout, with abv over 7%, are perhaps closest to the original in character.