The Imperial System

Every time I’m confronted with inches, pints and pounds (lb’s), I get confused. This anti-metric system is called “The Imperial System” and is used in most English speaking countries I believe (but see below).

The imperial system of measurement is defined as a system that originated in Britain and came to formal use in the early 19th century with the Weights and Measures Act of 1824 and 1878.
The metric system is a system of measurement that succeeded the decimalised system based on the metre that had been introduced in France in the 1790s.

Defining a system of measurement for any country is a way to formalize and generalize the usage of some specific units of measurement over others. For example, the units that are commonly used in the UK include imperial units such as inches, pounds, pint, gallon, so it became a part of their curriculum. While driving, cooking, shopping, the British use imperial units.

As opposed to this, in a country like India, the metric system is more commonly used. Though at school they learn all the systems of measurement, the main focus is on the metric system as it is more commonly used in everyday life.

To top it off: the imperial system of measurement is different from the units and the metric system used in the USA.

Gray colour: the countries not using the metric system (The United States, Myanmar, and Liberia)

Imperial System Units

Under the imperial system of measurement, we can measure length, weight, distance, height, and volume by using some specific units. Look at the list below for the different imperial system units:

Length/DistanceInches (in), Feet (ft), Yard (yd), Mile (mi)
Mass/WeightGrain (gr), Ounce (oz), Quarter (qr or qtr), Stone (st), Pound (lb), Ton (t)
Volumefluid ounce (fl oz), gill (gi), pint (pt), quart (qt), gallon (gal)
Areaacre, square miles, square feet, square inches

Imperial System Vs Metric System

Some countries use the imperial system while most countries use the metric system of measurement. After the US gained independence from the British, they decided to keep the imperial system of measurement but with some changes. They made their own US Standard system which is quite similar to the imperial system.
The metric system has its own popularity, because it is the easiest measurement system to be used.
It is based on the decimal system or the powers of 10.
In the table below are the differences of the imperial system vs metric system:

Imperial SystemMetric System
The imperial system is defined as the measurement system used in countries like the UK, Liberia, Myanmar, etc. that uses units like an inch, pound, ton, etc.The metric system is defined as the decimal system of units based on the meters, kilograms, and second as the units of length, mass, and time respectively. These are the SI units that stand for ‘Système International‘.
There is no specific pattern in the conversion units. In 1 foot, there are 12 inches, and in 1 yard, there are 3 feet, etc.It is based on the decimal system, as units are based on powers of 10. For example, 1 meter is equal to 100 centimeters, 1 km = 1000 meters, 1 liter = 1000 ml, etc.
Imperial system units: Inch, Yard, Foot, Mile, Pound, Ounce, Gallon, etc.Metric system units: Meter, Centimeter, Liter, Kilometer, Gram, Kilogram, Millimeter, etc.

Imperial System Chart

An imperial system chart will help you to understand the conversion of imperial system units to other imperial units and to metric system units as well.

 LengthMassVolume/Capacity
Imperial System Conversion1 ft = 12 in
1 yd = 3 ft
1 yd = 36 in
1 mile = 1760 yards
1 lb = 16 oz
1 ton = 2000 lbs
1 gallon = 4 quarts
1 gallon = 128 fl oz
1 quart = 2 pints
1 pint = 2 cups
1 cup = 8 fl oz
Imperial System to Metric1 in = 2.54 cms
1 ft = 30.48 cms
1 yd = 91.44 cms
1 yd = 0.9144 m
1 mile = 1609.34 m
1 mile = 1.6 kms
1 oz = 28.34952 grams
1 lb = 0.45359 kg
1 lb = 453.59237 grams
1 ton = 0.90718 tonnes
1 gallon = 3.785 liters
1 fl oz = 29.57 ml
1 cup = 236.59 ml
1 pint = 473.176 ml
1 quart = 0.946 liters

Length

The international yard is defined as exactly 0.9144 metres. This definition was agreed on by the United States, Canada, the United Kingdom, South Africa, Australia and New Zealand through the international yard and pound agreement of 1959, and corresponds with the previous 1930s British and American definitions of 1 inch being 25.4 mm. In all systems, a yard is 36 inches.

The US survey foot and survey mile were maintained as separate units for surveying purposes to avoid the accumulation of error that would follow replacing them with the international versions, particularly with State Plane Coordinate Systems. The choice of unit for surveying purposes is based on the unit used when the overall framework or geodetic datum for the region was established, so that – for example – much of the former British empire still uses the Clarke foot for surveying.

The US survey foot is defined so that 1 meter is exactly 39.37 inches, making the international foot of 0.3048 meters exactly two parts per million shorter. This is a difference of just over 3.2 mm, or a little over one eighth of an inch per mile. According to the National Institute of Standards and Technology, the survey foot will be obsolete as of January 1, 2023, and its use discouraged.

Are you still there?

The main units of length (inch, foot, yard and international mile) were the same in the US, though the US rarely uses some of the intermediate units today, such as the (surveyor’s) chain (22 yards) and the furlong (220 yards).

At one time, the definition of the nautical mile was based on the sphere whose surface is the same as the Clarke Ellipsoid1. While the US used the full value of 1853.256 meters, in the Commonwealth, this was rounded to 6080 feet (1853.184 m). These have been replaced by the international version (which rounds the sixtieth part of the 45° to the nearest metre) of 1852 metres.

Weight and mass

Traditionally, both Britain and the US used three different weight systems: troy weight for precious metals, apothecaries’ weight2 for medicines, and avoirdupois weight3 for almost all other purposes. However, apothecaries’ weight has now been superseded by the metric system.

One important difference is the widespread use in Britain of the stone of 14 pounds (6.35029318 kg) for body weight; this unit is not used in the United States, although its influence was seen in the practice of selling flour by a barrel of 196 pounds (14 stone) until World War II.

Another difference arose when Britain abolished the troy pound (373.2417216 g) on January 1, 1879, leaving only the troy ounce (31.1034768 g) and its decimal subdivisions, whereas the troy pound (of 12 troy ounces) and pennyweight are still legal in the United States, although these are no longer widely used.

In all these systems, the fundamental unit is the pound (lb), and all other units are defined as fractions or multiples of it. The tables of imperial troy mass and apothecaries’ mass are the same as the corresponding United States tables, except for the British spelling “drachm” in the table of apothecaries’ mass. The table of imperial avoirdupois mass is the same as the United States table up to 1 pound, but above that point the tables differ.

Are you still there?

The imperial system has a hundredweight, defined as eight Stone of 14 lbs each, or 112 lb (50.80234544 kg), whereas a US hundredweight is 100 lb (45.359237 kg).
In both systems, 20 hundredweights make a ton. In the US, the terms long ton (2240 lb, 1016.0469088 kg) and short ton (2000 lb; 907.18474 kg) are used to distinguish them. Further, the term metric ton is used to denote a tonne (1000 kg, 2204.62262 lb), which is about 1.6% less than the long ton.

The US customary system also includes the kip, equivalent to 1000 pounds force, which is also occasionally used as a unit of weight of 1000 pounds (most usually in engineering contexts).

Absolute madness!


1 In 1887 the English surveyor Colonel Alexander Ross Clarke CB FRS RE was awarded the Gold Medal of the Royal Society for his work in determining the figure of the Earth. 

2 The apothecaries’ system, or apothecaries’ weights and measures, is a historical system of mass and volume units that were used by physicians and apothecaries for medical recipes and also sometimes by scientists.

3 The avoirdupois system is a measurement system of weights that uses pounds and ounces as units. It was first commonly used in the 13th century AD and was updated in 1959. 

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