The units that have no single value
This converter carries only units with a single verifiable value. The ones below have several, with no objective reason to prefer one: putting one of them in the catalogue would mean choosing silently on the reader's behalf.
The dunam, from the Ottoman dönüm to a thousand square meters
The modern dunam is 1000 square meters, and that is the one the converter carries: Turkey, Israel and Jordan have used this metric form since the early twentieth century.
The Ottoman dönüm it descends from was about 919.3 square meters, and that value is still alive in the West Bank and Gaza, where deeds continue to quote it. Iraq settled on a third value, 2500 square meters. A figure in dunams therefore means nothing without its country.
The bigha, which changes with the Indian state (katha)
Uttar Pradesh and Bihar count the bigha at about 2529 square meters, West Bengal and Assam at 1337.8, and Rajasthan knows two, the pakka bigha at 2529 and the kaccha bigha at 1618. The largest is more than twice the smallest.
The smaller units scatter the same way: the katha is one twentieth of a bigha, so between 67 and 126 square meters depending on the state. Indian deeds are legally valid only in square meters or acres, and an advertisement in bighas is always read by first asking which bigha.
The marla, a hundred and sixtieth of an acre split by a border (sarsahi)
The marla is not a local measure but a fraction of the acre: the British revenue survey of Punjab set it at one hundred and sixtieth, that is 272.25 square feet or 25.2929 square meters, with twenty marla to the kanal and eight kanal to the acre. The whole ladder was built to close exactly on the acre, and that is where such an awkward value comes from.
The rounding to twenty-five square yards adopted after 1947 broke that closure: a hundred and sixty marla of that size make 3344.5 square meters, that is 0.826 acre and not a whole one. Below it the sarsahi carries the division by nine further, 2.81 square meters on one side of the border and 2.32 on the other, and it is still printed on the revenue records of both Punjabs without the document always saying which of the two it counts.
The tatami, and the Japanese word that names two quantities
The area of a Japanese home is counted in jō, that is in tatami mats, but the tatami has no single size: 1.5488 square meters in the Edo format, 1.6562 in the Chūkyō one, 1.8241 in the Kyōto one. Property rules settle it from below by requiring at least 1.62 square meters per jō advertised.
The chō raises a different problem: the same word names a length of 109.09 meters and an area of 9917.36 square meters. A catalogue cannot carry two quantities under one symbol, and it is context, never the unit, that says which is meant.
The tan, a sixth taken off in 1594, and a bolt of cloth under the same word (tanmono)
The tan is 300 bu, that is 300 tsubo or 991.74 square meters, and it is one tenth of the chō. Under the ritsuryō system it was 360 bu, 1190.08 square meters, and Hideyoshi's cadastral survey brought it down to 300 in 1594. The cut is one sixth, and that is where the mistake is made: on the ground it reads the other way round, since a paddy field nobody has touched counts a fifth more tan after the survey than before. A figure in tan therefore says nothing without the date of the deed that carries it.
That survey did not measure land to know its area but to turn it into rice: every tan was given a yield rating, one and a half koku for the best paddy and one and a tenth for the poorest, and the sum of those ratings made the declared worth of a domain. The tan is the unit through which an area became a fortune. The same word also counts cloth, with no connection whatever to the paddy field: a tan of kimono cloth, the tanmono, is a bolt about 36 centimeters wide and some ten meters long, the amount needed to cut one adult garment, and that is the form in which the word is still printed on labels.
The catty, the tael and the Zentner (Doppelzentner, Pfund)
The catty is 604.79 grams in Hong Kong, where it is defined by law, 500 grams in mainland China since metrication, and 600 grams in Taiwan. The tael is one sixteenth of it and scatters the same way: the Hong Kong tael, 37.7994 grams, is still used to quote gold.
The German Zentner is 50 kilograms, the Austrian and Swiss one 100. The German Doppelzentner is also 100 kilograms, so the same weight is a Zentner on one side of the border and a Doppelzentner on the other. Only the Pfund survived that confusion, at exactly 500 grams, and that is the one the converter carries.
The Brazilian alqueire and the Korean geun (braça, momme)
All three alqueires descend from the braça of 2.2 meters, and that is their only kinship: the mineiro is a hundred braças square, 48,400 square meters, the paulista fifty by a hundred, 24,200, exactly half, and the northern one seventy-five square, 27,225. A ten-alqueire holding therefore doubles or halves depending on the state it sits in.
The Korean geun poses the same problem in another quantity: it weighs 600 grams for meat and fish but 375 grams for fruit and vegetables, in the same market on the same day. The don, by contrast, is 3.75 grams with no argument, which is why its Japanese equivalent is in the converter under the name momme.
The qirat, which weighs, which measures and which measures nothing (feddan, sahm)
One word names three unrelated quantities. To a jeweller the qirat is a mass, the metric carat of 0.2 gram. In Egypt it is an area, one twenty-fourth of a feddan, or 175 square meters. In an Egyptian title deed it is not a quantity at all but a share: a whole property counts twenty-four qirats whether it covers thirty square meters or three hectares.
The sahm carries the same division further, five hundred and seventy-six sahms making up the whole property, and Cairo deeds are commonly written in qirats and sahms rather than in percentages. Measured against the feddan, a sahm is 7.29 square meters. That twenty-fourth is all the three uses have in common, and it is also the twenty-fourth of gold fineness: taking the word qirat without asking which quantity is meant means being right once in three.
The Spanish vara, and the land measures that grew out of it (cuerda, manzana, caballería)
The vara never had a single length: 0.8359 meter for the Castilian vara, about 0.838 in California, and exactly thirty-three and a third inches in Texas, that is 0.8467 meter, a value Texas land descriptions still use. Every land measure built on its square inherits that spread.
The Puerto Rican cuerda is 3930.4 square meters, or 0.971 acre, and it remains the unit of the island's deeds although everything else there is measured the American way. The Central American manzana is ten thousand square varas, or 6988.96 square meters. The caballería runs from 13.42 hectares in Cuba to 78.6 in Puerto Rico, where it is two hundred cuerdas: the same word, almost six times the area.
The arpent, a length on one side of the Atlantic and an area on the other
The arpent is a length in North America and an area in France, and the word does not say which. The linear arpent is 180 French royal feet, that is 58.47 meters or 191.8 English feet; the arpent of area is its square, 3418.9 square meters, about 0.845 acre. The old Paris arpent, measured 220 French feet to a side, reaches 5107, half as much again.
It did not die with the old regime. Louisiana, Mississippi, Alabama and Florida still recognise it in their deeds, at 0.84628 acre, that is 3424.8 square meters: six square meters more than the Quebec arpent, under the same name. Mauritius and the Seychelles still sell land by the arpent, but by theirs, 4220.87 square meters. And the royal edict of 1716 can still be read on the map of Louisiana, which handed out land two to four arpents of frontage by forty to sixty deep: hence those narrow strips running back from the river for more than two kilometers, which the American grid never managed to erase.
The Morgen, which measures a morning of ploughing rather than an area (Joch, Tagwerk)
The Morgen does not measure a surface but a piece of work: what a team turns over in one morning with a single-share plough. The measure follows the soil and the tool, never geometry. Its values once ran from 1906 to 11780 square meters depending on the region. After metrication, Prussia and Hesse settled it at 2500 square meters, twenty-five ares, the value everyday German still means; Baden counted 3600 and Swabia 3152. Austria and Bavaria say the same thing in other words, the Joch at 5755 square meters and the Tagwerk, literally a day's work, at 3407.
The word travelled further than its values. The Dutch morgen followed the settlers to the Cape, and the British colonies of the Cape and Natal as well as the Boer republics of the Transvaal and the Orange Free State kept it, fixed at 8565.32 square meters, 0.8565 hectare or 2.117 acres. That is three and a half times the Prussian Morgen under the same name, and it still appears in South African and Namibian title deeds, where old areas were never rewritten in hectares.