In Ancient Roman measurement, congius (pl. congii, from Greek konkhion, diminutive of konkhē, konkhos, "shellful") was a liquid measure that was about 3.48 litres (0.92 U.S. gallons). It was equal to the larger chous of the Ancient Greeks. The congius contained six sextarii.

Cato tells us that he was wont to give each of his slaves a congius of wine at the Saturnalia and Compitalia.Pliny relates, among other examples of hard drinking, that a Novellius Torquatus of Mediolanum obtained a cognomen (Tricongius, a nine-bottle-man) by drinking three congii of wine at once:

The Roman system of weights and measures, including the congius, was introduced to Britain in the 1st century by Emperor Claudius. Following the Anglo-Saxon invasions of the 4th and 5th century, Roman units were, for the most part, replaced with North German units. Following the conversion of England to Christianity in the 7th century, Latin became the language of state. From this time on the word "congius" is simply the Latin word for gallon. Thus we find the word congius mentioned in a charter of Edmund I in 946.