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 is that system of units of measure in use in the United States of America since the United States' ratification of the Convention on the Meter. As a matter of law, all US Customary units are defined in terms of their metric counterparts.
- 1 Length
- 2 Area
- 3 Weight
- 4 Mass
- 5 Liquid Volume
- 6 Power
- 7 References
The units of length in the US Customary system are the inch, the foot, the yard, and the mile.
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.
The US Customary foot, originally the length of an adult human foot, has a length of twelve inches. In fact, the US Customary foot is much longer than the average length of a human foot today, although some persons have feet that measure longer than this.
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 at least three other units of length that are found in Elizabethan-era literature:
- The fathom is the length of a rope held between two outstretched arms. It is defined as six feet. It may be used as a unit of depth at sea, but the usual unit of depth at sea 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 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.
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.
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.
One ton is two thousand pounds.
Originally the English system defined the hundredweight (abbreviated cwt) as one hundred pounds. This is slightly heavier than an ancient talent.
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.
One fluid pint is the volume of water that weighs one avoirdupois pound. As such, it measures sixteen fluid ounces.
One quart is the volume of two pints.
One gallon is the measure of four quarts.
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.