Butt (unit) - Units - Tun

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 US gallons (950 l; 210 imp gal), but occasionally other sizes (e.g. 256 gal., 240 gal., and 208 gal.) 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." —Untitled manuscript, consisting of a list of various customs duties, dated 15 July, 1507

Originally, the tun was defined as 256, or (28), gallons; this is the basis for the name of the quarter of 64 corn gallons. At some time before the 15th Century, it was lowered to 252 wine gallons, so as to be evenly divisible by small integers, including seven, since 252=22×32×7. Note that a 252-gallon tun of wine weighs about 2,240 pounds, the same as a unit of weight (not volume) that is known today as the long ton or imperial ton.

With the adoption of the Queen Anne wine gallon of 231 cubic inches (3in×7in×11in), the tun became exactly 58,212 cu in, which is approximately the volume of a cylinder with both diameter and height of 42 inches (assuming π≈22⁄₇). This gallon was adopted as the standard US liquid gallon – therefore, using the standardized international inch (of 25.4 mm), the US tun would be exactly 953.923769568 litres.

When the imperial system was introduced the tun was redefined in the UK and colonies as 210 imperial gallons. Since 210=2×3×5×7 the imperial tun remained evenly divisible by small integers. There was also little change in the actual value the tun; assuming the current definition of the imperial gallon, the tun would be exactly 954.6789 litres (only ~0.0792% larger than the US tun).