US Customary System: An Origin Story

Measuring tape shows inches part of the US Customary System

The United States’ uniqueness is reflected in omnifarious facets of life. Just look at units of measurement. While most nations use the metric system, the US retains, for the most part, its own standardized system, often, and erroneously, referred to as the Imperial system. In truth, the US uses the US Customary System, which was standardized decades before the British Imperial System. However, the two have similar roots that have fed into their shared units.

How Units of Measurement Were Created

Units of measurement have been around since the early days of civilization. They were, much like everything else at the time, birthed out of necessity. Without shared units to define quantifiable goods, there is no basis for commercialism or trade. Therefore, units of measurement were needed as populations swelled and measurements were needed for common exchanges. They also helped in comprehending the world around us.

Traditionally, archaic units of measurement were based on the parts of the body. This makes sense for several reasons. Firstly, when you are perceiving the natural world, the easiest method of engaging with space is with your body parts, specifically your hands and arms. Furthermore, these units would be similar person-to-person, considering that people of similar backgrounds and builds would be performing similar construction or other tasks requiring measurements. However, there likely were issues with accuracy. It is also important to consider that measurements with the arms and hands simplified the entire process, as you always had your measuring tool with you.

The Giza pyramids in Cairo were built using cubits

Some early units of measurement included the digit (the width of the finger, now about 0.75 inch), the palm (width of the palm, now about 3 inches), the span (width of the outstretched hand, from the tip of the thumb to the tip of the finger, or 3 palms or 9 inches), and the cubit (length of the forearm, approximately 18 inches.)

These practices fed into what would eventually become British Imperial and US Customary units, but the manner in which they developed is dependent on the varied history and peoples of Ancient Britain.

History of English Units

During the Bronze Age, the Celtic Britons infiltrated modern-day Britain. A lot of cultural detail has been lost about the Celts, but it is possible that some of their measurement units influenced the modern systems. Starting around 450 CE, Germanic tribes invaded England, displacing the Briton population.

With these tribes in control, Anglo-Saxon England saw distance measurements that have persisted until today. The inch (ynce) was the length of 3 barleycorns, an amount that actually is remarkably close to its current length. They also used several foot measurements, with one being equal to 12 inches, but another, which consisted of two shaftments, equaled 13 inches. The Saxon unit for area was the acre, which also retains its usage in almost all English-speaking countries. The word “acre” literally meant “field,” and this unit was considered a field the size that a farmer could plow in a single day.

In historical England, various units of weight were related as multiples of the grain, which was originally the weight of one barleycorn. For volume, the gallon was utilized. Originally, it was the volume of eight pounds of wheat. Larger volumes of liquids may have been carried in hogsheads, a unit of volume that unsurprisingly has vanished from common use.

In 1066, the Normans invaded England. Hailing from France and having a blend of Norse Viking, Frankish, and Gallo-Roman ancestry, the Normans retained a lot of Saxon culture, such as units of measurement. All land in England was measured traditionally by the rod (gyrd), an old Saxon unit about equal to 20 feet. 40 rods made a furlong (fuhrlang), the length of a traditional furrow plowed by ox teams on Saxon farms. Norman kings fixed the length of the rod at 5.5 yards, which is still unchanged today.

The Normans also brought numerous Roman units to England. Please note, however, that the Romans invaded England throughout Celtic times, so these units of measurement may have been acquired through numerous means.

For example, the mile is a Roman unit, originally defined as the length of 1000 paces of a Roman legion. Eventually, in 1592, the British Parliament set the length of a mile at 8 furlongs—5280 feet.

The Normans also brought to England the Roman tradition of the 12-inch foot. When this became official, Norman King Henry I ordered the construction of 3-foot standards, which were called yards. In fact, according to legend, King Henry I decreed that the yard was the distance from the tip of his nose to the end of his outstretched thumb. In reality, however, this is unlikely, as both the foot and the yard were based on the Saxon ynce, the foot at 36 barleycorns and the yard 108.

The pound was also a Roman measurement, originally at 12 ounces, but it was later shifted to 16 after a preference by European merchants.

Ultimately, Saxon, Norman, and Roman influences shaped the English units, as they were called. These were used in both Great Britain and the American Colonies.

Standardization of Units in the Early United States

However, there was an even wider influence on American units of measurement than there was in Britain. Even before the American Revolution, the colonists were faced with a hodgepodge of units of measurement from England, Holland, France, and Spain. This brought a great deal of confusion.

This is exemplified by the bushel. Measurements weren’t always the same between colonies. In Connecticut, a bushel was 28 pounds. In New Jersey, a bushel was larger, at 32 pounds. Ultimately, after independence, the states developed uniform weights and measurements. This birthed the US Customary System.

Differences Between US Customary and British Imperial Units

To add to the divide between formerly English units, the British Imperial System was established in 1824. This made some specific changes to the existing units from which the US system had derived. Furthermore, the Mendenhall Order of 1893 defined the US yard and pound, as well as related US Customary units, in terms of the metric meter and kilogram. Therefore, there is no longer any direct relationship between US Customary and Imperial units of the same name.

Regardless, the US Customary and British Imperial Systems remain almost identical. The most substantial differences are found in volume. There are differences in the following:

  • The British Imperial fluid ounce is equal to 28.413 milliliters, while the US Customary fluid ounce is 29.573 ml.
  • The British Imperial pint is 568.261 ml (20 fluid ounces), while the US Customary pint is 473.176 ml (16 fl oz).
  • The British Imperial quart is 1.13 liters (40 fl oz), while the US Customary quart is 0.94 L (32 fl oz).
  • The British Imperial gallon is 4.54 L (160 fl oz), while the US Customary gallon is 3.78 L (128 fl oz).
British Pint

Throughout the 1900s, the United Kingdom underwent significant metrification. In result, the Imperial System’s official usage in the UK is confined to the above units for volume, as well as miles per hour (MPH) for vehicle speed. In the US, the US Customary units retain usage for commercial and everyday purposes. However, in both nations, the metric system generally is heralded for scientific measurements.

US Customary Units

Even though the US Customary and Imperial Systems are not used internationally, there is a need to comprehend their equivalents in the metric system. Some common conversions include 1 yard=0.9144 meter, 1 lb=0.45359237 kilogram, 1 joule=1 watt second, and 1 Newton=0.224809 pound force.

Please note that mechanical units in inches (e.g. for fasteners) are sometimes referred to colloquially as SAE units. This, of course, derives from ANSI-accredited standards developing organization SAE International, which traditionally used the US Customary System. However, SAE has since switched to metric for specifications in its standards.

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4 thoughts on “US Customary System: An Origin Story
  1. The statement “The pound was also a Roman measurement, originally at 12 ounces, but it was later shifted to 16 after a preference by European merchants” is simplisitic. England had a large number of different pounds – by the 1970’s these had been whittled down to two – the Apothecaries pound (12 Apothecaries ounces) of 5760 grains and the Avoirdupois pound (16 ounces) of 7000 grains.

  2. It might be appropriate to record that the Mendenhall Order defined the yard as being 3137/3600 metres exactly. This differed from the British definition by a few parts per million. In 1960, the British, American, Australian and SOuth Africna standards laboratories agreed on the “Intenrational Yard” which was defined as 0.9144 metres exactly. This differed from the older US definition by abot two parts per million. The older US definition, which became known at the “Survey yard”, is to deprecated in 2023.

  3. Excellent summary & historical perspective of USC units. How could I get a pdf version of this article for training purpose ? Pl advise. Thanks.

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