http://www.gwydir.demon.co.uk/jo/units/length.htm

Length and Area

Length
 12 lines       = 1 inch 12 inches    = 1 foot 3 feet          = 1 yard 1760 yards  = 1 mile  36 inches   = 1 yard  440 yards   = quarter mile  880 yards   = half mile 100 links       = 1 chain  10 chains     = 1 furlong    8 furlongs  = 1 mile     4 inches    = 1 hand   22 yards      = 1 chain   5.5 yards     = 1 rod, pole or perch  40 poles       = 1 furlong

The units in common use were inches, feet, yards and miles. We all had to know our twelve times table! An inch is the width of a thumb, a foot is the length of a foot (!) and a yard is a single stride, all for a large man. My thumb is 3/4 inch across, my foot is 11 inches and my stride is 2 feet.

The foot has been used for over a thousand years, the inch since medieval times, and the yard in the reign of Henry I  (1100-1137) as within a tenth of an inch of the modern yard.

The inch is defined as exactly 2.54 centimetres. This means that the Imperial units of length are based on the metric system!

A mile is derived from "mille", Latin for thousand, since a mile is a thousand Roman double paces, from left foot to left foot, about 5 feet, which would make 5000 feet. The mile is 5280 feet. In the past every part of England had its own mile, up to 2880 yards (it is now 1760 yards). In ireland, the mile was 2240 yards well into the 20C.

A chain is the length of a cricket pitch. It has been used since 1620.

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. The perch was used in the reign of Henry II (1154-1189), the pole since the 16C, and the rod since 1450.

A furlong is a "furrow long" or length of a mediaeval field. It is used for the lengths of some horse races.

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 = 1 foot (which sounds slightly odd).

A line has been used since the 17C. It is used by botanists to describe the size of plants (which must be very small!) It is not common (in fact, I had never heard of it until I started researching this site!)

A thou is a colloquial term for a thousandth of an inch. It has been used since Victorian times.

Area
 4840 square yards   = 1 acre 1 furlong x 1 chain    = 1 acre 1210 square yards     = 1 rood  1 furlong x 1 pole     = 1 rood   4 roods                   = 1 acre 144 square inches    = 1 square foot    9 square feet           = 1 square yard    640 acres                = 1 square mile

Rods, poles, perches and roods were all rather confused. They could all be a measure of length (5.5 yards). Rods, poles and erches could also be a measure of area (5.5 yards square, or 30.25 square yards). So a 10 perch allotment would be 5.5 yards wide by 55 yards long. A rood could be a measure of area (1210 square yards). The dictionary also cheerfully states that this could vary round the country!

While I am not giving metric tables (they're BORING), a hectare is 10,000 square metres (or a hundredth of a square kilometre).

Older measures

There are some measures that were not in our "tables" which we had to learn as children, because they had fallen into disuse (although I have never heard of anyone using rods, poles or perches!) However, you do read about them. Many imperial measures of length were originally taken from parts of the body, sometimes obviously, such as hands or feet. But then at some time a standard
length was decided on, and used by everyone. These older measurements, however, are not so precise.

A span is the length from your little finger to your thumb if you stretch your fingers, or 9 inches (23 cm).

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 (46 cms), but the Romans, Egyptians and Hebrews all had different lengths. Cubits are used in the Bible. The ark was 300 cubits long. An ell is derived from "elbow". It started off similar to the cubit (see above), but the English ell was 45 inches long (115 cm). Other countries had different lengths for their ell. There was an old saying "Give him an inch and he'll take an ell", which got changed to "Give him an inch and he'll take a mile". A cloth-yard was used to measure cloth. It is 37 inches long (94 cm), which is an inch longer than an ordinary yard. A cloth-yard shaft was an arrow a cloth-yard long. A league is another measure that varies by country. In England, it is taken to be 3 miles. 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 7 leagues (10.5 miles) in one stride! Julys Verne wrote a book about a submarine, called "40,000 Leagues under the Sea" (this refers to how far they travelled, not how deep they were!). A hide was enough land to support a house-hold, usually between 60-120 acres (24-48 hectares). A hide of good land was smaller than that of poor quality. Hides are used in the Doomsday Book. An acre is 4840 square yards (see above) and is a conventional measure of area. It was defined in the time of Edward I (1272-1307) and was supposed to be the area that a yoke of oxen could plough is a day. Acre is derived from the Latin for field. But the common field system of medieval times in Britain was ten acres. An acre is a furlong (furrow length) long and a chain wide. In fact, an archaic word for furlong was "acre-length" and for chain "acre-width". The Scottish and Irish used to have different values for their acres. The Scottish acre was 6150.4 square yards and the Irish acre was 7840 square yards.

Some archaeologists have deduced a megalithic yard from the statistical study of prehistoric stone circles, which they estimate to be 2.72 feet. It seems strange that pre-historic man had such a consistent measure, when in the past the mile was defined differently in different parts of England! Surely, they just measured out their circles with paces, which tend to be that length! It doesn't have to be a formal unit.