Following is an explanation of the Imperial System, the USA system, and their history. This is part of a set of pages on the metric, USA and Imperial systems of measurement; to see the main page (and other related pages), click on Metric System Conversion.
Aside from the metric system, there are two other main systems of measurement in use today: the Imperial system of measurement and the USA system of measurement. The two systems are often confused with each other and sometimes one or the other terms are used to collectively refer to both systems (in particular, Europeans often use the term Imperial system to refer to either systems). However, despite similarities between the two, they are different systems. To properly explain their relationship, one needs to consider the history behind them.
Both the Imperial system and the USA system of measurement are based on the older English units of measurement. Prior to 1066 the English units included AngloSaxon measures (likely of ancient Germanic origin). After the Norman conquest (Battle of Hastings in 1066), the Normans reintroduced Roman measures, resulting in a system including ancient AngloSaxon measures and Roman measures. These units were standardised by the Magna Carta of 1215 and were periodically reviewed and updated (notably in 1496, 1588 and 1758). The UK Weights and Measures Act of 1824 was one such review, which not only modified the types and sizes of the units, but renamed them from the English units of measurement to the Imperial System of Measurement. This new standard was then introduced throughout the UK and its colonies at this time.
However, the USA had become independent prior to this and consequently did not adopt the Imperial system of measurement. Instead, they had developed their own measurement standards, based on the English units system which was used throughout the States prior to independence. As a result, the older English units developed into the USA system of measurement in the USA and into the Imperial system of measurement in the UK and countries colonised by the UK.
Although the measurements in the metric system are derived from scientific principles, the English units measurements (and the subsequent USA and Imperial measurements) are based on nature and everyday activities. For example, a league is based on the distance that can be walked in an hour. Sailors (in days gone by) would drop a weighted rope into the water, lowering it by lengths (where each length was measured by holding the rope between their outstretched hands) until the weight at the end of the rope touched the seabed. This led to the definition of the fathom as the distance from the fingertips of one hand to the fingertips of the other, when the hands are held straight out to the sides. A grain (used to measure small quantities of precious metals) is the weight of a grain of wheat or barleycorn.
Such natural measures were well suited in a simple agricultural society. However, as trade and commerce grew, it was necessary to have more consistent measures (after all, not all grains of wheat have the same weight and not all sailors have the same length of arm). Consequently, metal weights and lengths were produced to represent exact measures; these metal representations where then used to produce official scales and measurements to ensure that trade was based on standard quantities. For larger measures (e.g. a mile) it was impractical to build a metal equivalent, so they were redefined to be multiples of the smaller measures. It is for this reason that the mile was changed in 1595 under Queen Elizabeth I's reign from the Roman standard of 5000 feet to 5280 feet (which is 8 furlongs, each furlong equal to 10 chains, each chain equal to 22 yards and each yard equal to 3 feet).
Despite the development and standardisation of the English units of measure, their roots in ancient agriculture and trade have resulted in a diverse and relatively complex set of measurements. The various trades each developed their own measures, so in many cases the measure would depend on what it was being used for: a barrel of oil is not the same size as a barrel of wine (there are in fact eight different barrel sizes officially recognised). Likewise there are both fluid ounces and weight ounces, with different types of weight ounces (depending on what was being weighed). This complexity was not eliminated when the English system evolved into the Imperial and USA systems, with the result that these systems have approximately 300 different units of measurement. In comparison, the metric system has only 7 basic units of measurement (which can be increased or decreased in multiples of 10 to make larger or smaller units, or combined to make more complex units).
For a list of the most common measures and their relationships to each other, click on Table of Metric and Imperial units.
Development of USA and Imperial Systems
For many years the USA and the UK continued to develop their measurement standards, with little or no coordination between the two. As a result, differences in the units evolved and the two systems became increasingly different despite both having English units as a common base. For example, the USA pint has 16 ounces whereas the Imperial pint has 20 ounces. In addition, there are a number of units which exist in one system but not in the other.
Since the mid20th century, the USA and UK standards organisations have worked together to bring the two systems closer together. In part, this is due to the impact of globalisation and the resulting need to have common measurements. It is also due to the growing acceptance that the metric system is now the international standard for measurement. An example of both influences can be seen by the decision in 1958 (implemented in 1959) for the USA yard and the UK yard to be jointly redefined to be equal to 0.9144 meters.
From the mid20th century all major countries using the Imperial system have been gradually replacing the Imperial system by the metric system. This is a complex transition as it involves not only reeducating the population, but also considerable changes to machinery and production equipment. In the UK this transition is largely done, with the only two major Imperial units still in general use being the pint and the mile.
In the USA there has been considerable debate about replacing the USA system of measurement by the metric system. Currently there is no plan to do so. Consequently, the USA is the only major country that has retained a nonmetric system (the other two countries are Liberia and Burma).
For further information on the history of the Imperial and USA systems converting to the metric system of measurement, click on Metric System & History.
Advantages of the Metric System
The metric system is a relatively modern system (just over 200 years old) which has been developed based on scientific principles to meet the requirements of science and trade. As discussed above, the Imperial and USA systems have evolved without any such constraints, resulting in a complex set of measurements that fit everyday life in a simple agricultural society but which are unsuited to the requirements of science and modern trade. Consequently, the metric system offers a number of substantial advantages:
 Simplicity. The Metric system has only 7 basic measures, plus a substantial number of measures using various combinations of these base measures. The imperial system (prior to the UK converting to metric) and the USA system have over 300 different measures (see http://lamar.colostate.edu/~hillger/traditional.htm), of which many are ambiguous. For example, there are eight different definitions of ton (including the 'short ton' and the 'long ton'), all of which differ in weight. There are also eight different definitions of barrel, five different definitions of bushel, three different definitions of mile (international, nautical, and US Survey). The USA gallon is smaller than the Imperial gallon, and all measures which are multiples or subdivisions of the gallon (e.g. pint, quart) likewise are inconsistent between the USA and the UK. Not only are the USA and UK pints different in size, but there are also differences between 'dry pints' and 'liquid pints'.
 Ease of calculation. All the units in the metric system are multiplied by 10 (to make larger units) or divided by 10 (to make smaller units). For example a kilometer is 1000 meters (10 * 10 * 10). It's nearest equivalent is a mile which is 5280 feet (8 * 10 * 22 * 3; based on the calculation that a mile is 8 furlongs, 10 chains to a furlong, 22 yards to a chain, 3 feet to a yard). Although complex calculations can be done using the Imperial or USA system, almost all calculations can be done easier and faster in the metric system.
 International Standard. With the exception of the USA, all major countries have converted to the metric system (although in some countries, such as the UK, the conversion to metric is not yet complete). Consequently, for any international communication (trade, science, etc.) the metric system is the most widely used and accepted.
Conversion to and from Metric
To convert between these 3 systems (metric, USA, Imperial), click on Quick Metric Converter. For more detailed information, click on Metric System Conversion, which provides several metric system converters and conversion tables.
Historical Trivia
One of the measures in the USA Measurement System is the Mark Twain. It is defined as the minimum safe clearance for steam wheel boats and is set to 2 fathoms. This was subsequently used as the pen name of the famous American writer (who lived during the time of steam wheel boats and featured then in some of his writings).
The Cannot Shot measure is approximately 3 miles in length and is the distance originally used for US territorial waters.
The Nautical Mile was previously defined as 1 minute of arc along the Meridian of the Earth. However, as the Earth is not perfectly round (flattened at the poles) this equals 6108 feet at the North or South pole and only 6046 feet at the equator. Thus, the length of the nautical mile depends on where your are! In the 20th century this inconsistency was removed by redefining the nautical mile to be exactly 1852 meters.
A much more extensive list of historical explanations can be found at Weights and Measures Evolution
