Tun (unit)

The tun (Old English: tunne, Latin: tunellus, Middle Latin: tunna) is an English unit of liquid volume (not weight), used for measuring wine, oil or honey. Typically a large vat or vessel, most often holding 252 wine gallons, but occasionally other sizes (e.g. 256, 240 and 208 gallons) were also used.

In one example from 1507, a tun is defined as 240 gallons.

Early Modern English: "He that ys a gawner owght to understonde there ys in a tunne lx systerns and every systern ys iiii galons be yt wyne or oylle."
Translation: "He that is a gauger ought to understand that there is in a tunne 60 sesters, and every sester is 4 gallons, be it wine or oil."

The word tun is etymologically related to the word ton for the unit of mass, the mass of a tun of wine being approximately one long ton, which is 2240 pounds (1016 kg).

Originally, the tun was defined as 256 wine gallons; this is the basis for the name of the quarter of 64 corn gallons. At some time before the 15th century, it was reduced to 252 gallons, so as to be evenly divisible by other small integers, including seven.

With the adoption of the Queen Anne wine gallon of 231 cubic inches the tun approximated the volume of a cylinder with both diameter and height of 42 inches. These were adopted as the standard US liquid gallon and tun.

When the imperial system was introduced the tun was redefined in the UK and colonies as 210 imperial gallons. The imperial tun remained evenly divisible by small integers. There was also little change in the actual value the tun.

Standard tuns of wine came to serve as a measure of a ship's weight capacity.

In the US customary system, the tun (symbol: US tu) is defined as 252 US fluid gallons.

...
Wikipedia