Units of Measure index

Imperial Measures of Length

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This page gives Imperial units of length. Square lengths, such as acres, are on the area page, cubic lengths are on the volume page and speed and acceleration are on the other units page. There is a quick convertor below which gives an answer rounded to 2 decimal places. The metric equivalents on this page are also all to 2 decimal places. There are more advanced conversions on the tables page.

Enter value:   =   ?  

Well-known units - inch, foot, yard, mile
Land measurement - furlong, chain, link, rod, pole, perch, Ramsden's chain
Nautical units - fathom, shackle, cable, nautical mile
The human body - foot, hand, palm, nail, finger
Old units - span, cubit, ell, bolt, cloth-yard, league, megalithic yard, pyramid inch
Small units - line, thou, barleycorn, poppy seed

Well-known units - Everyone knew these and used them constantly.
inchin or "2.54 cm12 inches = 1 foot The inch was originally the width of a thumb. The name comes from uncia which is Latin for 'twelfth part' (see foot). An inch is considered to be the width of a thumb (my thumb is 3/4 inch wide). Click here for some foreign units called 'thumbs'.
In England, the inch has been in use since medieval times. In 1324, Edward II decreed that the inch was the length of 3 barley corns placed end-to-end. The inch is the basis of the Imperial measures of length, and is now legally defined as exactly 2.54 centimetres. This means that the Imperial units of length are based on the inch worm metric system! If you want a quick conversion, then 2 inches is roughly 5 centimetres.
To inch forwards means to move slowly forwards. An inch worm is a small caterpillar which moves forwards by bending its body into a loop, then straightening, looking as if it's measuring something. "Inchworm, inchworm, measuring the marigolds.."
footft or '30.48 cm12 inches = 1 foot
3 feet = 1 yard
There was a Roman unit called a pes (plural pedes) which means a foot, and was 29.59cm, which is nearly the size of the modern foot. There were twelve uncia to a pes as well (see inch).
The foot has been used in England for over a thousand years. The foot, a length of the human foot, was anything from 9 3/4 to 19 inches (my foot is 11 inches). It was not until 1844 that there was anything resembling a real standard. In that year the British government created a standard master yard in a length of bronze, marked off in feet and inches. Click here for a photo of a similar one in Trafalgar Square. School rulers in Britain are usually a foot long (30 cm). The ruler below is 6 inches or half a foot. Click here for an old-fashioned wooden ruler.
6 inch ruler
A light year is the distance that light travels in a year. The nearest stars are a few light years away. A foot is approximately a light nanosecond! (A nanosecond is a billionth of a second, or 1/1,000,000,000 secs.)
Beware something called a metric foot, used by DIY stores. It is 30 centimetres long, which is less than a foot, and you don't get a whole number of them to a metre. So it is useless for both imperial and metric measurement, a typical British compromise!
yardyd91.44 cm3 feet = 1 yard
1760 yards = 1 mile
A yard is a single stride (my stride is 2 feet). The word yard comes from the Old English gyrd, meaning a rod or measure. Henry I (1100-1135) decreed the lawful yard to be the distance between the tip of his nose and the end of his thumb. It was within a tenth of an inch of the modern yard. A yard is nearly a metre. The picture below shows the end of a yard rule.
 yard rule
milemi or m1.61 km1760 yards = 1 mile A mile is derived from mille, Latin for thousand, since a Roman mile was mille passuum, a thousand Roman paces or double strides, from left foot to left foot. A passus was 5 pedes (see foot), which would make 5000 feet to the mile. The modern mile is 5280 feet or 1760 yards. In the past every part of England had its own mile, up to 2880 yards. In Ireland, the mile was 2240 yards well into the 20C.
At school, we had to learn that half a mile was 880 yards, and quarter of a mile was 440 yards. People still say "about a hundred yards" to mean a short walking distance.
Note that 'm' is the abbreviation for both a mile and a metre! But 'mi' means mile as well. Still, road signs in the United Kingdom continue to use 'm' as the abbreviation for mile.
The conversion rate between a kilometre and a mile is very approximately the Golden Ratio (about 1.6). The Golden Ratio is a mathematical constant - the limit of the ratio of successive Fibonacci numbers. It also occurs in pleasant proportions in art and architecture. This is entirely coincidence and doesn't mean anything, but it might help some mathematicians with the conversion!
Statutory Instrument 1995 No. 1804 allows the use of the mile, yard, foot or inch for road traffic signs, distance and speed measurement in Britain.

Land measurement - Although most people would not use these, for some reason they were in the school tables that we had to learn. They were used by surveyors, but they also crop up in descriptions of land, such as allotments. Click here for land measurement of area.
furlong201.17 m8 furlongs = 1 mile
10 chains = 1 furlong
220 yards = 1 furlong
A furlong is a 'furrow long' or length of a medieval field (see acre). The furlong was also known as flatt, furshott, or sheth. The length varied depending on the type of soil. It was usual for horses to take a short breather at the end of the furrow before turning in to the next furrow. The heavier the soil, the harder the horses had to work, and so the less time between breathers. This led to shorter furrows or furlongs between the ends, and therefore local variations in furlong length. These variations were not removed until the railways required a universal standard measurement.
Furlongs are used for the lengths of some horse races. They used to be the measurement for human races as well.
chain20.12 m10 chains = 1 furlong
100 links = 1 chain
22 yards = 1 chain
A chain is the length of a cricket pitch. It has been used since 1620. Its correct name is a Gunter's chain or surveyor's chain, since it was invented by the Rev. Edmund Gunter (1581-1626), a professor of astronomy at Gresham College, London. There is a different chain called the Ramsden's chain.
A correspondent says "The whole of the United States was measured and mapped using the Gunters Chain and his chain still applies to all title plans in use today.  For this reason all city blocks, roads and avenues are multiples of the chain. Towns were laid out at 6 miles square or 36 sq miles. Early farms were sold to would-be farmers as lots of 640 acres or 1 sq mile. Interestingly enough the Geodetic coastal survey and ordnance surveys of the entire US are metric."
Another correspondent says "Many of the older people in Jamaica use chains as a measure of distance. 'The shop is about 5 chains down the road' for example. The only other useage I have ever encountered is on railways, where the radius of curvature of a line used to be measured in chains."
link20.12 cm100 links = 1 chain
7.92 inches = 1 link
Links and chains are so-called as distances were measure with real links, made of metal. In the story Kim, written by Rudyard Kipling, there are several references to links and chains:
So far as Kim could gather, he was to be diligent and enter the Survey of India as a chain-man...
Colonel Creighton Sahib - this was unfair - sent Kim a written examination-paper that concerned itself solely with rods and chains and links and angles...
Since Mohammedan horse-boys and pipe-tenders are not expected to drag Survey-chains round the capital of an independent Native State, Kim was forced to pace all his distances by means of a bead rosary.
rod, pole or perch5.03 m40 poles = 1 furlong
4 poles = 1 chain
5.5 yards = 1 pole
1 rod = 1 pole = 1 perch
Rods, poles and perches are different names for the same unit. Medieval ploughing was done with oxen, up to 4 pairs at a time. The ploughman handled the plough. His boy controlled the oxen using a stick, which had to be long enough to reach all the oxen. This was the rod, pole or perch. It was an obvious implement to measure the fields, such as 4 poles to the chain. A BBC webpage about allotments says that "an allotment plot is 10 poles" and claims that "A pole is measured as the length from the back of the plough to the nose of the ox". I suppose that if you wanted to control the front ox, you needed a pole long enough to reach! The perch was used in the reign of Henry II (1154-1189), the pole since the 16C, and the rod since 1450. In the 16th century the lawful rod was decreed to be the combined length of the left feet of 16 men as they left church on a Sunday morning. In North Devon there is a tradition that fencing, that is to say the cutting and laying of a hedge, would be done at so much a land yard, which seemed to be about 5 paces or 5.5 yards, which would equate to a rod, pole or perch. An earlier name for a rod was a gyrd which is the derivation of a yard.
A correspondent told me: "In the early 1960s my mother had come into possession of a small cottage in Bwlch y Cibau (mid Wales). She wanted the hedge round the garden laid and made stock proof. A local man (in his late 50s I think) quoted her as three and six a rod." See money for what "three and six" means - it's 17.5 pence in modern money!
Ramsden's chain30.48 m100 feet = 1 Ramsden's chain The Ramsden's chain or engineer's chain is 100 feet long, where each link is one foot long (see below).
chain chain
I've had a suggestion that this chain was called after Ramsden was Jesse Ramsden (1735-1800), an English astronomical and scientific instrument maker.
A correspondent writes:
"From my own experience using this chain, it was used for geographically surveying land. On longer stretches, two people, one at each end of the chain, would 'flip flop' end for end. One person would remain stationary while the other walked past the first until the chain reached its tether producing a count of unit measure and then usually an iron peg placed at the desired length to be marked off. These are called surveyors' stakes."
See also Gunter's chain.

A correspondent suggests: "You might care to add to your bit about measuring chains that these expanded or contracted with heat. When George Everest and his minions surveyed India with chains they had to measure the air temperature all the time and calculate the changing length of the chain. I think I read in a biography of the man that India was mapped within a two inch tolerance! And they demolished whole buildings and cut down trees in order to keep triangulation lines dead straight. Everest, apparently, got very uptight when people didn't pronounce his name as Eve-rest: so he must be spinning in his grave every time someone talks of Mount Everest! There's karma for being a cantankerous old fogey."

Nautical units - I must confess that I know nothing of sailing, so I hope that I haven't made too many mistakes in the following!
Click here for Points of the Compass, click here for Beaufort wind force scale and click here for knots.
fathom1.83 m6 feet = 1 fathom
15 fathom = 1 shackle
Fathoms measure depth of water. They have been in use in England since before 1600, and may be derived from faethm, the Anglo Saxon word for 'to embrace' (because it is roughly the distance from one hand to the other if your arms are out-stretched). There was a similar unit in Scandinavia and Germany.
Shakespeare writes in The Tempest:
          Full fathom five thy father lies;
          Of his bones are coral made:
          Those are pearls that were his eyes:
          Nothing of him that doth fade,
          But does suffer a sea-change
          Into something rich and strange.

Fathoms were also used to measure the depth of coal seams and Cornish mines. Dolcoath Mine in Camborne was one of the deepest with a level at 420 fathoms or 2520 feet vertical drop. There is a local saying about a person who keeps him or herself to themselves as being "as deep as Dolcoath".
To fathom something means to throroughly understand it or get to the bottom of it. This could come from taking a sounding from a ship, which would be measured in fathoms.
shackle27.43 m15 fathom = 1 shackle
30 yards = 1 shackle
Shackles were a measure of the length of cable. According to a 19th Century Seamanship Manual, ships were usually equipped with 12 shackles of bower cable where each shackle was a 12.5 fathom length. These 12.5 fathom lengths were joined by shackles (hence the name) and by swivel links to overcome twisting. When paying out the anchor cable, counting the number of shackles passing gave a measure of the length used. In 1949, the Royal Navy switched from 12.5 fathom shackles to 15 fathom shackles. Modern heavy mooring chain is usually sold in 15 fathom lengths or 'shots' (an American term). The specification sheets quote the number of links per shot. Here is a formula for calculating the amount of anchor chain to put out in your ship: Twice the square root of the depth of water in fathoms = the number of shackles of cable.
cable185.32 m608 feet = 1 cable
10 cables = 1 nautical mile
The cable is a tenth of a nautical mile. Since the nautical mile has altered, so has the cable. The metric equivalent (left) is based on the old Imperial length of 608 feet. The cable seems to be a measure of length along the surface, rather than a measure of depth, or even the length of an anchor cable. An anchor cable will always be an integral number of shackles long, but the unit cable is 6.76 shackles. In 1970, the RN Hydrographic Charts Dept. switched their nautical mile from 6080 feet to 1852 metres (the International NM). At the same time, their cable became a tenth of the International Nautical Mile.
nautical mile1.85 km6080 feet = 1 nautical mile
10 cables = 1 nautical mile
Nautical miles measure distance. 1 nautical mile is the angular distance of 1 minute of arc on the earth's surface. As these differ slightly (6108' at pole and 6046' at equator) 6080 was adopted (this being its approximate value in the English Channel). The International nautical mile is 1852 metres, so is very slightly different from the UK nautical mile (1853.18 metres). The metric equivalent on the left is in kilometres to 2 decimal places, which ducks the whole problem. Now the International nautical mile is used throughout the UK, except in the Statutory Instrument 1995 No. 1804 which defines it as 1853 metres.
If you want an irrelevant fact, one minute of arc on Mars is close to a kilometre (.987 km to be more accurate). Perhaps the French who defined the kilometre were really Martians!
The kilometre, like the nautical mile, was also defined as one minute of arc on a great circle on the earth. However, it was one minute of arc in the new French revolutionary decimal system, in which a circle was made up of 400 degrees, each of 100 minutes. Thus the conversion from kilometres to nautical miles was k=M*(400*100)/(360*60). Of course disputes over the standard size of a great circle on the earth meant it wasn't absolutely exact, but it's exceedingly close!
NASA describes the International Space Station as orbiting at an altitude of 211 nautical miles.
A knot is a nautical mile per hour.

The human body - Of course many Imperial units were based on the human body, but these are called after parts of the body.
foot30.48 cm12 inches = 1 foot see above
hand10.16 cm4 inches = 1 hand
3 hands = 1 foot
Hands are used to measure horses. You measure from the ground to the withers of the horse (its shoulder) since it won't keep its head still. 3 hands is 1 foot (which sounds slightly odd).
palm11.43 cm3 inches = 1 palm
4 palms = 1 foot
A palm was 3 inches. A hand is an inch bigger. Possibly the idea was that a hand was the width of the hand including the thumb, and a palm was the width excluding the thumb.
nail5.72 cm16 nails = 1 yard A nail was 2 and a quarter inches. It was a cloth measure. Thinking about it, this can't be a human nail as it's too large. It must be a small metal nail.
finger11.43 cm8 fingers = 1 yard A finger was 4 and a half inches. Another cloth measure.

Older units - these belong further back in history, I think, and the lengths often varied.
span22.86 cm9 inches = 1 span
4 spans = 1 yard
A span was originally the length from your little finger to your thumb if you stretch your fingers. It later became 9 inches or a quarter of a yard.
cubit45.72 cm18 inches = 1 cubit
2 cubits = 1 yard
construction of Noah's Ark, from H. Schedel's Nuremburg Chronicle of 1493 The cubit is the earliest unit of length, used in Egypt in the 3rd Dynasty (2800-2300 BC). It is the length of the arm from the elbow to the tip of the middle finger. The English cubit is 18 inches long, but the Romans, Egyptians and Hebrews all had different lengths. Cubits are used in the Bible. The ark was 300 cubits long. See Scripture measures in the "Multum in Parvo" Table Book.
cloth-yard93.98 cm37 inches = 1 yard A cloth-yard was used to measure cloth. It is an inch longer than an ordinary yard. A natural way to measure cloth is to hold one end in one hand, and measure along the edge to the nose, then repeat, and these would be cloth-yards. I measure thread for making lace in the same way. A cloth-yard shaft was an arrow a cloth-yard long.
ell114.3 cm45 inches = 1 ell
5 spans = 1 ell
32 ells = 1 bolt
An ell is derived from 'elbow'. It started off similar to the cubit (see above), but the English ell was 45 inches or a yard and a quarter. It could have been measured from elbow to elbow. Other countries had different lengths for their ell. There was an old saying "Give him an inch and he'll take an ell". As the ell fell out of common use, the saying got changed to "Give him an inch and he'll take a mile" (which makes less sense).
bolt36.58 m32 ells = 1 bolt
40 yards = 1 bolt
Another measure of cloth. Cloth is stored in rolls, which are still called bolts.
league4.83 km3 miles = 1 league A league is another measure that varies by country. In England, it is taken to be 3 miles. It was originally the distance that you could walk in an hour. Tennyson wrote of the Charge of the Light Brigade
          Half a league, half a league, half a league onwards
          Into the valley of death rode the six hundred.

In fairy tales, there were seven league boots, which would carry you seven leagues (21 miles) in one stride. Jules Verne wrote a book about a submarine, called "20,000 Leagues under the Sea". This refers to how far they travelled, not how deep they were.
megalithic yard82.91 cm2.72 feet = 1 megalithic yard Some archaeologists have deduced a megalithic yard from the statistical study of prehistoric stone circles, which they estimate to be 2.72 feet. I can't see why pre-historic man had a consistent, formal unit, when in the more recent past the mile was different lengths in different parts of England. I think they just measured out their circles with paces which tend to be that length. If you are interested in the megalithic yard, see this website. I don't agree with it myself!
pramid inch2.54 cm1.001 in = 1 pyramid inch This is taken from a display in the Whipple History of Science Museum, in Cambridge:
"Charles Piazzi Smyth travelled to Giza in 1864 to measure the Great Pyramid. He had read and admired theories linking the Pyramid with revelations in the Old Testament concerning a 'sacred cubit' used by Noah. This unit, it was argued, was 1/20,000,000th of the earth's axis of rotation (distance pole to pole) and had been recorded in the shape of the pyramid as a material trace deliberately left by its ancient-Israelite builders. Piazzi Smyth sought to verify these theories, and link them to British imperial units of measurement.
The centrepiece of Piazzi Smyth's argument came from a striking correlation: the 'sacred cubit' he claimed to have recovered in his measurements of the Gread Pyramid was very close to being 25 inches long. This was proof, he argued, that the Anglo-Saxons had inherited their imperial measurements from God via the ancient Israelites - a powerful defence of the imperial system, then facing serious attack from the French metric system. Britain had only recently redefined the standard yard, and Piazzi Smyth boldly argued that it was in error and should be re-established using his 'pyramid inch', which equaled 1.001 standard british inches.
Piazzi Smyth's project failed. Sceptics reviewed his measurements and questioned his selected use of data. More tellingly, the Egyptologist Flinders Petrie soon found that the new dimensions taken of the Great Pyramid in 1880 were most easily explained using the Egyptians' own measure of length, the Egyptian cubit."

Small units - less than an inch!
line2.12 mm12 lines = 1 inch A line has been used since the 17C. It is used by botanists to describe the size of plants. It is not common (in fact, I had never heard of it until I started researching this site!)
pica4.23 mm12 points = 1 pica
6 picas = 1 inch
Used in printing. See font size.
point0.35 mm72 points = 1 inch
12 points = 1 pica
Used in printing. See font size.
thou25.4 microns1000 thou = 1 inch A thou is a colloquial term for a thousandth of an inch. It was introduced in 1844. It is generally credited to Joseph Whitworth who demonstrated that as little as 1/1000" was the difference between clearance and intereference. A thou is 0.0254 mm. Another name for 'thou' is the 'mil' (a 'milli-inch'), but 'mil' is also used an a measure of angle.
barleycorn8.47 mm3 barleycorns = 1 inch barley corns This is mentioned in this Tudor set of measures.
poppy seed2.12 mm4 poppy seeds = 1 barleycorn
12 poppy seeds = 1 inch
I was informed of this by a correspondent.

Finally, I can't resist adding this (half-inched from Twitter).

relationship between length units

They've left out 22 yards to a chain (Gunter's)! Everyone knows that! (We learned it at school.) How else would you know how long a cricket wicket is? (By the way, 'half inch' is rhyming slang. It means pinched,or stolen.)

Someone has pointed out the finger in that diagram may be wrong. Above, I've said that a finger was 4 and a half inches, and was a cloth measure. The diagram says that it's 7/8 inch. I suspect that this is a different unit. It could be to measure spirits in a glass. Someone would say "Give me two fingers of whisky", which would be the height in a glass of two fingers' width. The width of a finger would be about 7/8 inch. This is, of course, really a measure of volume, and would depend on the size of the glass. Or maybe the diagram's "finger" is something else altogether.

Someone else (my husband!) has pointed out a genuine error. It says a cable is 100 fathoms. A fathom is 6 feet or 2 yards. A cable is 608 feet or 101 and a third fathoms. Not even close! That is based on the old Imperial value of the nautical mile of 6080 feet. A modern nautical mile is 1852 metres (metric! boo hiss...) A modern cable would then be 185.2 metres or about 101.27 fathoms. Still wrong!