Medieval Muslims were capable of very accurate measurements. Precise measurements and calculations allowed them to build mosques with enormous domes, construct globes depicting the earth, and make numerous astronomical discoveries. Islamic merchants paid close attention to the details of their transactions, measuring out precise amounts of their products when making sales. At the same time, however, for many people, measurements did not need to be especially precise. It did not matter to them how Muslims in another country measured lengths or weights as long as everyone in their locality agreed on standards.
Weights and measures in the medieval world were far from standard. Although people throughout the Islamic region used similar measurements, often with similar names, the actual values of these measurements varied from time to time and place to place. Historians attempting to discern ancient and medieval measurements are often frustrated by the fact that these values were not consistent. Medieval systems of measurement evolved from ancient ones. The Islamic system developed out of existing Persian, Greek, and Roman systems.
The basic length measurement was the foot, which roughly corresponded to the Western foot. This measurement was originally made by placing a person's foot against the object being measured, but this method presented obvious problems of standardization. Once people began using sticks to measure lengths, the foot was standardized at about 12.6 inches. This foot then served as the basis for other measures. An arsh was between 1.5 and 2 Arabic feet. A dirha was about a cubit, or the length of a forearm. An orgye was 6 Arabic feet. A seir, which corresponded to the Latin stadium, was 600 Arabic feet. A farasakh was 18,000 Arabic feet, or about 3 to 4 miles, depending on the size of the foot. On the smaller end of the spectrum, a cabda was one-fourth of an Arabic foot, and an assba was 1/16th of an Arabic foot, or about the length of a finger.
Different places used different names and standards for their measurements. In Baghdad during the ninth and 10th centuries a unit called the habl was about 43 yards long, and 250 habl were equal to about 11,000 yards. One mil was equal to about 2,020 yards. A djarib was equivalent to a square habl, or about 1,900 square yards. Contemporary scholars computed that Baghdad's total area at the time was about 43,750 djarib.
To provide standards in weights and volumes, towns, merchants, and anyone else who was concerned about weights would own weights of a standard size and weight. Most standard weights were made of glass or bronze, though during the Fatimid era (9091171) some weights were made of lead. Historians have found some bronze weights in Egypt, many of them shaped like barrels with smooth sides or like octagons with faceted sides. These were similar in shape to contemporary Byzantine weights. Many weights were made with a small punch mark on one side bearing a legend of a weight's value, the name of the current ruler, and a guarantee. These standard weights could vary depending on the commodity being measured; for example, salt might be weighed against one standard, while wheat might be weighed against another. Standard sizes included 1 dirhem, 2 dirhems, 10 dirhems, and 1 ratl.
Many Arabic weights were made of glass, often shaped like coins. Governments maintained standards in weights and measures and in coinage, and often there was considerable overlap between the names and values of weights and coins. Glass coin weights were small, round, and flat with Arabic inscriptions on either side. Modern historians who unearthed medieval coin weights at first thought that they were used as currency but later realized that they were weights. These weights did, in fact, sometimes function as small change between the 10th and 13th centuries, especially when silver was in short supply.
Many Islamic measurements corresponded to earlier Greek and Latin units of measure. For example, the dirhem was a unit of weight in the Middle East. The dirham was also a coin. Historians have suggested that the names dirhem and dirham came from the ancient Greek coins and units of weight called "drachmas." The dirhem varied in weight by time and place. During economic depressions throughout the medieval period the value of the dirhem was debased, to less than 0.10 ounces. In some areas the dirhem was defined by the weights of seeds. In medieval Egypt a dirhem weighed as much as 60 husked barley seeds.
The awqiyya, or uqiya, was a unit of weight throughout the Islamic world. It corresponded to the ounce, or uncia in Latin. Its value varied by time and place; it usually weighed about 1 ounce, though it could weigh over 2 pounds. There were 12 uqiya to a pound. The pound weight was called a ratl in Arabic. It weighed 128 dirhems, or about 14.5 ounces, though this weight varied; historians have reported values for the ratl ranging from less than 1 pound to over 4 pounds. When a ratl weighed 14.5 ounces, an uqiya weighed about 1.2 ounces.
Weight measures were often defined by practical everyday units, such as numbers of seeds or commodities, such as dates. The kirat was 1/16 of a dirhem. It was equivalent to the weight of four barley seeds, approximately 0.007 ounces. This unit was the origin of the carat measurement for gemstones. The miskal or mithqal was the weight of 6,000 mustard seeds, or about 0.15 ounces. It was based on the Byzantine weight called the solidus. The gold coin known as the dinar corresponded with the miskal. A daniq was one-sixth of a dinar; it could also be one-sixth of a dirham. Pearl merchants used a weight called the methkal, which might have been related to the miskal, though historians do not know precisely how much it weighed. Pearl merchants also used units called the yeka and the rthi, which could have come from the Indian unit of weight known as the rati. Muslim pearl merchants imported many of their pearls from India, so it would not be surprising to find Indian units of measure in that trade.
In the Persian Gulf region dates were a common commodity, and the people of the area created several standard measures to quantify dates. These measures took the form of baskets or sacks woven from date palm fronds, and they could hold dried, pressed, or fresh dates. Because the sacks were made to the same size, merchants and customers could count on them holding more or less the right weight of dates. For example, a sack that was about 27.6 inches long and 15.7 inches wide could hold approximately 88 pounds of dates. This unit was known as a jirab. Towns and regions had their own local standards, and these could vary widely.
The main measurement in the Gulf region was the mun, which weighed about 9 pounds. The mun was divided into 24 kiyas, which weighed about 6 ounces each. A family of six needed 20 mun of wheat to survive for six months. That same family needed an additional 12 jirab of dates to make it through the rest of the year.
Measures of volume varied widely. The measure known as the qadah was similar to the liter or quart, though it was not standardized. A mudd also was about 1.8 pints, or perhaps a little more. An ardabb or irdabb was about 20 gallons. Grain and pulse merchants used a type of measuring cup or scoop called a cheeas to measure out precise amounts of wheat, lentils, or other dry items. The cheeas was a wooden scoop that held about 2.7 pints, or half a mun. There were also scoops that would parcel out half or one-quarter of a cheeas.