Imperial and US Customary Systems

Most people have heard of the imperial system of measurement, but are you familiar with the US customary system? The two systems are very closely related, and they both come from the same English system, which itself is a derivative of the measurements used by Ancient Romans, the Carolingians, and Saxons. In fact, the two systems are so closely related that there is often some confusion between them, since in some cases they share unit names, not sizes.

The units of measure that we use today were adopted around the time of the Norman Conquest. The yard is the only unit that has not really changed since that time. This measurement replaced the ell. The chain is another measure, which came from England and is relatively unchanged. Confusingly, the foot which is in use today is a modified version of the foot that was introduced back then, and that unit replaced the agricultural foot. Today, there are 16 and a half feet in a rod, but originally there was a nice round number – 15 agricultural feet. The furlong and the acre have not changed much over the last thousand years either. They were originally a measure of the value of land, but when they became more about size their measurements were fixed.

In England, there have been three types of pound in use – the troy pound, the apothecaries’ pound, and the avoirdupois pound. The troy and apothecaries’ pounds are made up of 5760 grains, or 12 ounces, while an avoirdupois pound is 7000 grains, or 16 ounces.

Perhaps the most unusual, however, is the units of volume. A fluid gallon in the United States contains 0.83 Imperial gallons, while a dry gallon contains 0.97 imperial gallons. In England, all gallons are measured as one imperial gallon.

Things become even more complex with the adoption of the metric system, which is meters and centimeters, and kilograms and grams. In England, it is not uncommon for heights to be measured in feet and inches, distances in miles, and weights in kilograms. Pressure is typically measured in pounds per square inch in both countries, even though the metric system has a special unit – the pascal – for this purpose.

After the US Declaration of Independence, the United States branched off and made its own system of units of measurements, and over time the UK and US ended up with different gallons, pounds and yards. Eventually, the two governments decided to work together to define an exact definition of the yard and the pound, based on copies of the official standards that the British Parliament adopted in the 1850s. It was later agreed that this ‘official’ standard was not of very high quality, so in 1960 the two governments redefined the pound and the yard based on the standards used for the kilogram and the meter. While the change was incredibly small, it resulted in there being two linear measurement standards in the United States – the Surveyor’s System, and the International System.

These discrepancies are a common topic of discussion among tourists. In the UK, drinks are often bought in pints, and a British Pint is larger than an American Pint, and the same is true for gallons. Queue jokes about Americans not being able to hold their liquor, and British being ‘ripped off’ with their petrol prices.

Before the 1960s, the imperial yard and the imperial pound were close enough to the US Customary system that for day-to-day use, especially short distances and the sale of, for example, food, the units were interchangeable. There were some differences in what was considered normal use, however. For example, the US convention is to measure short road distances using feet, while in the UK it is more common for yards to be used.

It’s hard to believe it, but there are people alive today who grew up with a different measurement system to the one that we use now. The old Imperial system used 14 pounds. The hundredweight was eight stone, and the ton is 2240 pounds, or 20 hundredweight. The Customary system does not use stones, but uses the hundredweight of 100 pounds, with a ton being 2000 pounds. This is easier to remember (something that is often cited as a benefit of the Metric system, where everything is measured in multiples of ten), but it has not been adopted in international trade. To help people from other countries understand references, when a ton is mentioned, if it is an Imperial ton it is called the ‘long ton’ while the customary ton is called the 'short ton'. Both measurements are accepted, as long as it is made clear what it is that the trade is working with.

If you think that the system is confusing today, spare a thought for those who lived in the 19th century. Thomas Jefferson noted in the “Plan for Establishing Uniformity in Coinage, Weights and Measures” that there were 14 different definitions for the gallon listed just in the English Statutes alone. Those gallons range from 224 cubic inches up to 282 cubic inches. Eventually, the Treasury picked the Queen Anne Gallon, which was the second smallest out of those gallons, to become the official measure.

To enable easy trading, the international commodity market uses the barrel as its main unit of measure for trading things like oil. A barrel is 159 liters, or 42 US customary gallons. Precious metals are traded in troy ounces, with an ounce weighing 31.10 grams.

Eventually, we may see the whole world agree on a system of measurement – which would most likely be the metric system. For now, however, we must work with this international mixture of measurements, where there are units with similar names and different values - even within the same country. Isn't the world a crazy place!

This is a guest article written by Jonathan Leger. Jonathan Leger is a freelance writer and small business owner.