The tun (Old English: tunne, Latin: tunellus, Medieval Latin: tunna) is an English unit of liquid volume (not weight), used for measuring wine,^{[1]} 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.^{[2]}
In one Early Modern English example from 1507, a tun is defined as 240 gallons.
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. 
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^{[2]}^{[3]}^{[4]} 
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).
History
Originally, the tun was defined as 256 wine gallons;^{[nb 1]} 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.^{[nb 2]}
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.^{[nb 3]} 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.^{[nb 4]} There was also little change in the actual value of the tun.^{[nb 5]}
Standard tuns of wine came to serve as a measure of a ship's weight capacity.^{[5]}





Definitions
In the US customary system, the tun (symbol: US tu^{[6]}) is defined as 252 US fluid gallons (about 954 litres).^{[6]}
In the imperial system, the tun is defined as 210 imperial gallons (about 955 litres).
Conversions
Both the imperial and US tuns were subdivided into smaller units as follows.
1 tun  ≡  2  butts or pipes 
≡  3  puncheons or tertians  
≡  4  wine hogsheads^{[6]}  
≡  6  tierces  
≡  8  wine barrels  
≡  14  rundlets  
Conversions of the imperial tun are as follows.  
1 imperial tun  ≡  210  imperial gallons 
≡  1680  imperial pints  
≡  954.6789  litres^{[nb 7]}  
≡  0.9546789  cubic metres^{[nb 7]}  
≈  69910  cubic inches  
≈  40.4570  cubic feet  
≈  252.199484  US fluid gallons  
≈  2017.595875  US fluid pints  
Conversions of the US tun are as follows.  
1 US tun  ≡  252  US fluid gallons^{[6]} 
≡  2016  US fluid pints^{[6]}  
≡  953.9237769568  litres^{[nb 8]}  
≡  0.953923769568  cubic metres^{[nb 8]}  
≡  58212  cubic inches  
≡  33.6875  cubic feet  
≈  209.833895  imperial gallons  
≈  1678.671156  imperial pints 

Explanatory notes
 ^ 256=2^{8}
 ^ 252 = 2^{2}×3^{2}×7
 ^ The volume, V, of this cylinder may be approximated from the height, h, and the radius, r, as follows.
V = πr^{2}h ≈ 22⁄7×(21 in)^{2}×42 in since π ≈ 22⁄7 = (2^{2}×3^{2}×7)×(3×7×11) cu in = 252×231 cu in
 ^ 210 = 2×3×5×7
 ^ The imperial tun is only about 0.0792% larger than the US tun assuming current definitions. Note that 5 imp gal ≈ 6 US gal.
 ^ The conversion to litres is approximate and given as a range to reflect the varying definitions of the gallon and the tun in terms of the gallon.
 ^ ^{a} ^{b} ^{c} The conversion to litres is exact assuming the current 4.54609litre definition of the imperial gallon.
 ^ ^{a} ^{b} ^{c} The conversion to litres is exact assuming the current 25.4millimetre definition of the international inch.
References
 ^ Cardarelli, F. (2003). Encyclopaedia of Scientific Units, Weights and Measures. Their SI Equivalences and Origins. London: Springer. pp. 49. ISBN 9781447111221.
 ^ ^{a} ^{b} Zupko, Ronald E. (1985). "A Dictionary of Weights and Measures for the British Isles: The Middle Ages to the Twentieth Century, Volume 168". Memoirs of the American Philosophical Society. American Philosophical Society. 168. ISBN 9780871691682.
Quoting Gras (1918), p. 706
 ^ Gras, Norman S.B. (1918). Early English Customs Systems. Cambridge. p. 706.
Quoting Forgon (1507)
 ^ Forgon, T. (15 July 1507). Untitled manuscript, consisting of a list of various customs duties. Reproduced at sizes.com.
 ^ "Naval Architecture for All". United States Bureau of Transportation Statistics. Retrieved October 13, 2008.. "Historically, a very important and standard cargo for European sailing vessels was wine, stored and shipped in casks called tuns. These tuns of wine, because of their uniform size and their universal demand, became a standard by which a ship's capacity could be measured. A tun of wine weighed approximately 2,240 pounds, and occupied nearly 60 cubic feet." (Gillmer, Thomas (1975). Modern Ship Design. United States Naval Institute.) "Today the ship designers standard of weight is the long ton which is equal to 2,240 pounds."
 ^ ^{a} ^{b} ^{c} ^{d} ^{e} Cardarelli, F. (2003). Encyclopaedia of Scientific Units, Weights and Measures. Their SI Equivalences and Origins. London: Springer. pp. 43. ISBN 9781447111221.
 ^ "wine barrel". Sizes. 20090202. Retrieved 20180327.