US Customary System of Units
- This article describes US Customary units. For instructions on how to convert between US Customary and metric units, see English and Metric Units.
The United States Customary System of Units (or more commonly within the United States - standard units), or, in the United Kingdom, Imperial units are units of measurement. It is one of two primary systems of measurement. The other, the metric system, enjoys widespread use outside of the United States. Only two countries have not officially adopted the metric system - Liberia and Burma.
As a matter of law, all US Customary units are defined in terms of their metric counterparts.
The units of length in the US Customary system are the inch, the foot, the yard, and the mile.
- 12 in = 1 ft
- 3 ft = 1 yd
- 5,280 ft = 1 mi
One inch was originally the length of three barley corns, which form a distance approximating the breadth of the bridge of an adult human's nose. Today the inch is defined in terms of the centimeter with 2.54 centimeters per inch. An inch is also one-twelfth of a foot.
The US customary foot, originally the length of an adult human foot, has a length of twelve inches.
The yard, originally the length measured from the tip of the nose to the tip of the third finger with the arm fully outstretched, is defined as thirty-six inches, or three feet.
The statute mile, originally intended to approximate the length of a Roman mile, is defined as 5,280 feet.
The nautical mile, or the distance on the sea corresponding to one minute of latitude at or near the equator, is defined as 6,080 feet. The nautical unit of speed is the knot, which is one nautical mile per hour.
The classical English system has several other units of length including these:
- The fathom is the length of a rope held between two outstretched arms. It is defined as six feet. It is used as a unit of depth at sea by British mariners, but the usual unit of depth at sea in the USA is the foot.
- The rod is the length of a queue of sixteen persons. By convention, the rod is defined as five and one-half yards, or sixteen and one-half feet. 320 rods would therefore make a mile. The rod is rarely if ever used as a measure nowadays.
- The furlong is one-eighth of a mile, or 40 rods. It is close in length to the Roman stadium, which was one-eighth of a Roman mile. It is used in horse racing
- The chain, one-eightieth of a mile or 66 feet. It is used in land measure and on the railways. Importantly, the distance between the sets of stumps on a cricket pitch is 1 chain.
Most US Customary units of area are the squares of the units of length. The chief exception to this is the:
One acre is 160 square rods. 640 acres make a square mile.
Weight and mass
The US Customary system uses Avoirdupois weight. Effective July 1, 1959, the US Customary avoirdupois pound has been set equal to the international pound, and this in turn is based on a prototype pound in the British Exchequer. All other units of weight derive from this prototype. To be specific:
One ounce is one sixteenth of a pound.
The grain is 64.79891 milligrams, and by convention one pound is 7000 grains.
The stone, a unit of 8 lb, is very commonly used in the UK as a measure of weight, especially of people and animals. It is common, for example, for a British person to express his or her weight as 9 stone 8 pounds (written 9st 4lb), meaning 130 lb or approximately 59kg.
Originally the English system defined the hundredweight (abbreviated cwt) as one hundred pounds. This is slightly heavier than an ancient talent. In the UK, 1 hundredweight (cwt) is now defined as 8 stone, i.e. 112 lb; the hundredweight is still quite widely used as a measure of weight in the UK.
One US (or "short") ton is 2,000 pounds. An Imperial ("long") ton is 20 hundredweight or 160 stone or 2,240 pounds. The latter is being replaced by the metric ton (tonne) of 1,000 kg, or about 2,207 pounds.
In recognition that weight, being a force, is not an acceptable substitute for the measure of mass, physicists have added a mass unit to the US Customary system: the slug.
One slug is that mass which, when subjected to a force equivalent to a one-pound weight, will accelerate by one foot per second per second. Thus a pound is that force required to accelerate one slug of matter one foot per second per second. A slug would weigh thirty-two pounds on the ground, because the acceleration due to gravity at ground level on earth is thirty-two feet per second per second.
One fluid ounce is the volume of water that weighs one avoirdupois ounce.
In the USA, one fluid pint is the volume of water that weighs one avoirdupois pound. As such, it measures sixteen fluid ounces. In the UK, one pint is 20 fluid ounces.
In both the USA and the UK, one quart is the volume of two pints. This means that in the USA, 1 qt. is 32 fl.oz. but in the UK, 1qt. is 40 fl.oz.
In both the USA and the UK, one gallon is the measure of four quarts. This means that in the USA, 1 gallon is 128 fl.oz. but in the UK it is 160 fl.oz.
The usual US Customary unit of power is the foot-pound per second, or that power that does, in one second, the work of exerting a one-pound force over one foot. However, by convention, the US Customary system has a much larger unit of power:
The horsepower (abbreviated hp) is the power that an average horse could exert. One horsepower is 550 foot-pounds per second. Engines and motors often have their power rated in horsepower, as a direct measure of the size of a team of horses that would be required to match that engine or motor for power.