Perch (unit)

A perch is a unit of measurement used for area in the English system of measurement, and an ancient unit of height and volume in a number of systems of measurement. Its name derives from the Ancient Roman unit, the pertica.

Origin

The word perch is from the French perche, derived from the Latin pertica, meaning "a pole or staff". Originating in Roman antiquity, it spread with the Roman Empire and was likely re-introduced to England with the Norman conquest of 1066. In the Roman Empire, France and England, it also could mean area (square perches), and among operative masons of the Middle Ages, volume.

Area

As a unit of area, a square perch (the perch being standardized to equal 16​¹⁄₂ feet, or 5​¹⁄₂ yards) is equal to a square rod, or 0.00625 acres, or 1/160 acre. There are 40 square perches to a rood (A rectangular area with edges of one furlong (10 chains i.e. 40 rods) and one rod respectively), and 160 square perches to an acre (an area one furlong by one chain (i.e. 4 rods)). This unit is usually referred to as a perch or pole even though square perch and square pole were the more precise terms. Confusingly, rod was used as a unit of area but it meant a rood.

However, in the traditional French-based system in some countries, 1 square perche is 42.21 square metres.

As of August 2013 perches and roods are used as government survey units in Jamaica. They appear on most property title documents. The perch is also in extensive use in Sri Lanka, being favored even over the rood and acre in real estate listings there.

Perches were informally used as a measure in Queensland real estate until the early 21st century, mostly for historical gazetted properties in older suburbs.

Length

The perch as a lineal measure in Rome, was 10 feet (3.05 m), and in France varied from 10 feet (perche romanie) to 22 feet (perche d'arpent —apparently 1/10th of "the range of an arrow"—about 220 feet). To confuse matters further, by ancient Roman definition, an arpent equalled 120 Roman feet.

In England, the perch was officially discouraged in favour of the rod as early as the 15th century; however, local customs maintained its use. In the 13th Century Perches were variously recorded in lengths of 18 feet (5.49 m), 20 feet (6.1 m), 22 feet (6.71 m) and 24 feet (7.32 m); and even as late as 1820, a House of Commons report notes lengths of , 18 feet (5.49 m), 21 feet (6.4 m), 24 feet (7.32 m), and even 25 feet (7.62 m). In Ireland, a perch was standardized at 21 feet (6.4 m), making an Irish chain, furlong and mile proportionately longer by 27.27% than the "standard" English measure.

The rod as a survey measure was standardized by Edmund Gunter in England in 1607 as one-fourth of a chain (of 66 feet (20.12 m)), or long.

Volume

A traditional unit of volume for stone and other masonry. A perch of masonry is the volume of a stone wall one perch () long, 18 inches (45.7 cm) high, and 12 inches (30.5 cm) thick. This is equivalent to exactly .

There are two different measurements for a perch depending on the type of masonry that is being built:

a.) A dressed stone work is measured by the 24.75 cubic foot perch () long, 18 inches (45.7 cm) high, and 12 inches (30.5 cm) thick. This is equivalent to exactly .

b.) a brick work or rubble wall made of broken stone of irregular size, shape and texture, made of undressed stone, is measured by the () long, 12 inches (30.5 cm) high, and 12 inches (30.5 cm) thick. This is equivalent to exactly 16.5 cubic feet (0.611111 cubic yards; 0.467228 cubic metres).

See also

• Systems of measurement
• History of measurement