For other uses see Furlong (disambiguation).

A furlong is a measure of distance in imperial units and U.S. customary units equal to one-eighth of a mile equivalent to 220 yards 660 feet 40 rods or 10 chains. The exact value of the furlong varies slightly among English-speaking countries.

Five furlongs are approximately 1 kilometre (1.0058 km is a closer approximation). Since the original definition of the metre was one-quarter of one ten-millionth of the circumference of the Earth (along the great circle coincident with the meridian of longitude passing through Paris) the circumference of the Earth is about 40000 km or about 200000 furlongs. Contents 1 History 2 Use 3 Conversion to SI units 4 References History Farm-derived units of measurement: The rod is a historical unit of length equal to 5 yards. It may have originated from the typical length of a mediaeval ox-goad. The furlong (meaning furrow length) was the distance a team of oxen could plough without resting. This was standardised to be exactly 40 rods. An acre was the amount of land tillable by one man behind one ox in one day. Traditional acres were long and narrow due to the difficulty in turning the plough. An oxgang was the amount of land tillable by one ox in a ploughing season. This could vary from village to village but was typically around 15 acres. A virgate was the amount of land tillable by two oxen in a ploughing season. A carucate was the amount of land tillable by a team of eight oxen in a ploughing season. This was equal to 8 oxgangs or 4 virgates.

The name furlong derives from the Old English words furh (furrow) and lang (long). Dating back at least to the ninth century it originally referred to the length of the furrow in one acre of a ploughed open field (a medieval communal field which was divided into strips). The system of long furrows arose because turning a team of oxen pulling a heavy plough was difficult. This offset the drainage advantages of short furrows and meant furrows were made as long as possible. An acre is an area that is one furlong long and one chain (66 feet or 22 yards) wide. For this reason the furlong was once also called an acre's length though of course in modern usage an area of one acre can be any shape.

The furlong was historically viewed as equivalent to the Roman stade (stadium) which in turn derived from the Greek system. For example the King James Bible will use the term "furlong" in place of the Greek "stadion" whereas modern translations will translate into miles in the main text and relate the original numbers in footnotes.

In the Roman system there were 625 feet to the stade eight stade to the mile and three miles to the league. A league was considered to be the distance a man could walk in one hour and the mile (mille meaning one thousand) consisted of 1000 passus (paces 5 feet or double-step).

After the fall of Rome Medieval Europe continued with the Roman system which proceeded to "diversify" leading to serious complications in trade taxation etc. Around the turn of the century of 1300 England by decree standardized a long list of measures. Among the important units of distance and length at the time were foot yard rod(or pole) furlong chain and mile. The rod was 1/4 chain (25 links) 5 yards or 16 feet ( 3 feet/yard 5 yards) and the mile was 8 furlongs so the definition of the furlong became 40 rods and that of the mile became 5280 feet ( 8 furlongs 40 rods/furlong 16 feet/rod).

A 1675 description states "Dimensurator or Measuring Instrument whereof the mosts usual has been the Chain and the common length for English Measures 4 Poles as answering indifferently to the Englishs Mile and Acre 10 such Chains in length making a Furlong and 10 single square Chains an Acre so that a square Mile contains 640 square Acres." ---John Ogilby Britannia 1675

The official use of furlong was abolished in the United Kingdom under the Weights and Measures Act of 1985 which also abolished from official use many other traditional units of measurement. Use The 5 furlong (1006 m) post on Epsom Downs

Today the unit enjoys very limited use with the notable exception of horse racing measurements. Distances for thoroughbred horse races in Australia were metricated in 1972 1 but in the United Kingdom2 Ireland Canada and the United States races are still given in miles and furlongs. The city of Chicago's street numbering system allots a measure of 800 address units to each mile in keeping with the city's system of eight blocks per mile. This means that every block in a typical Chicago neighbourhood (in either North/South or East/West direction but rarely both) is approximately one furlong in length. Salt Lake City's blocks are also each a square furlong in the downtown area. However the blocks are irregular in shape as one moves further out from the centre but the numbering system remains the same (800 units to each mile) everywhere in Salt Lake County. Bus stops in Ann Arbor Michigan are about a furlong apart. Blocks in central Logan Utah and in large sections of Phoenix Arizona are similarly a square furlong in extent (eight to a mile which explains the series of freeway exits: 19th Ave 27th 35th 43rd 51st 59th ...). City blocks in Melbourne's Hoddle Grid are also one furlong in length. The furlong is also a base unit of the obscure FFF system of units. Conversion to SI units The exact conversion of the furlong to SI units varies slightly among English-speaking countries. In Canada3 and the United Kingdom4 which define the furlong in terms the international yard of exactly 0.9144 metre a furlong is 201.168 m. Australia5 does not formally define the furlong but does define the chain and link in terms of the international yard. In the United States which defines the furlong chain rod and link in terms of the U.S. survey foot of exactly 12003937 meter6 a furlong is approximately 201.1684 m; the US does not formally define a "survey yard". The difference of approximately 2 parts per million between the US value and the "international" value is insignificant for most practical measurements. References "Melbourne Cup 2010 Carnival Horse Racing". Worldwide Ticketing. 2010. http://www.worldwideticketing.com/horse-racing/melbourne-cup.htm. Retrieved 10 June 2010.  Example of the use of furlongs in horse racing Weights and Measures Act R.S.C. 1985 as amended; Schedule II Canadian Units of Measurement. Weights and Measures Act 1985 as amended; Schedule 1 Part VI Definitions of certain units which may not be used for trade except as supplementary indications. National Measurement Regulations 1999 Statutory Rules 1999 No. 110 as amended Schedule 11 Conversion Factors. NIST Special Publication 811 Guide for the Use of the International System of Units (SI) Appendix B B.6 U.S. survey foot and mile. National Institute for Standards and Technology U.S. Department of Commerce 2008.