List of unusual units of measurement

An unusual unit of measurement is a unit of measurement that does not form part of a coherent system of measurement; especially in that its exact quantity may not be well known or that it may be an inconvenient multiple or fraction of base units in such systems. This definition is deliberately not exact since it might seem to encompass units such as the week or the light-year which are quite "usual" in the sense they are often used; if they are used out of context, they may be "unusual", as demonstrated by the Furlong/Firkin/Fortnight (FFF) system of units.

One rack unit (U) is 1.75 inches (44.45 mm) and is used to measure rack-mountable audiovisual and computing equipment. Rack units are typically denoted without a space between the number of units and the 'U'. Thus a 4U server enclosure (case) is seven inches (177.8 mm) high.

The hand is a non-SI unit of length equal to exactly 4 inches (101.6 mm). It is normally used to measure the height of horses in some English-speaking countries, including Australia, Canada, the United Kingdom, Ireland and the United States.

The light-nanosecond was popularized as a unit of distance by Grace Hopper as the distance which a photon could travel in one thousand-millionth of a second (roughly 30 cm or one foot): "The speed of light is one foot per nanosecond." In her speaking engagements, she was well known for passing out light-nanoseconds of wire to the audience, and contrasting it with light-microseconds (a coil of wire 1,000 times as long) and light-picoseconds (the size of ground black pepper). Over the course of her life, she found many uses for this visual aid, including demonstrating the waste of sub-optimal programming, illustrating advances in computer speed, and simply giving young scientists and policy makers the ability to conceptualize the magnitude of very large and small numbers.

A metric foot (defined as 300 mm) has been used occasionally in the UK but has never been an official unit.

Horses are used to measure distances in horse racing – a horse length (shortened to merely a length when the context makes it obvious) equals roughly 8 feet or 2.4 metres. Shorter distances are measured in fractions of a horse length; also common are measurements of a full or fraction of a head, a neck, or a nose. "Neck and neck" is an example of a linguistic Siamese twin which refers to the neck as a unit of measurement in horse racing.

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