The SI unit for lengths and distances is the metre (m) (using international spelling or meter, U.S. spelling). Originally intended to be defined as one ten-millionth of the distance from the Earth's equator to the North Pole. The current definition is revised to be the distance light travels during 1/299,792,458th of a second. Each unit is a magnitude of ten compared to the metre. See the article on the metric system for more information and history about the metre.
The Ångström, named after the Swedish physicist Anders Jonas Ångström, is in contrast to the other metric units not officially part of the SI system, but is a negative 10th power of the metre and is often convenient to use on the scale of atoms, molecules and chemical bonds. One Å is about the size of the covalent radius of an atom of chlorine or sulfur.
The British (Imperial) system of measurements can trace it origins back to Roman, Saxon and Norman units. It is closely related to the US customary units but has gone through a reform in 1824 since its common roots why there are discrepancies between some of the units with the same name in the British and US customary systems. See the article on British (Imperial) And US customary systems to see the inter relations between the units which are less obvious than the metric system. For example a yard is three feet, while an inch in a 12th of a feet and so one.
However, since 1959 the United States together with UK and the Commonwealth countries has agreed upon an international measure, standardizing the common four measurements inch, foot, yard and mile which are presented here.
Most of these maritime units are linked to the British (Imperial) system as well with some exceptions. These are less precise since the mathematics don't really hold up to the definitions. A fathom should equal two yards (6 feet) despite at the same time being 1/1000 of a nautical mile which would imply it to be 6.08 feet.
To measure in space there has been need to invent new types of units suited to the immense distances. Parsecs are based on measuring the parallax of a star when observing it from the earth and from an hypothetical observer at the Sun. The inverse of the number of arc seconds of parallax is the number of parsecs to that star (the less parallax the further away the star must be).
A light year is not a time unit but a length since it is defined as the the distance light travels in vacuum during a Julian year (see the Time section for explanation of Julian vs. other types of years). The light minute and light second are defined in the same way. For reference, the distance to the Moon is 1.282 light seconds while the distance to the Sun is 8.3 light minutes.
The astronomical unit was originally defined as the mean distance between the Earth and the Sun but has been refined with a new definition to improve the accuracy, but the distance is the same.
A subset of the older English units are part of the standardized Imperial system of units. Here is a further set of units which has been used historically. For more information on these units and the ethymology and background behind the names see the article on old English length units.
Verkmått is an older Swedish system of units. The units have been defined by royal decrees in 1739 and a refined system in 1855 also called the decimal system. Sweden converted to the metric system in 1889 whereas the older units only has survived in niched areas like the dimensions of planks. The first version of verkmått uses the number 12 as base in contrast to the newer system after 1855 which like the metric system uses 10 as base. As example of this, there are 12 tum (inch) in a fot, and 12 linjer (lines) in a tum in the older version of the system. The units are annotated below with a (10) or (12) to indicate the version of the system.
The Shakkan-hō (尺貫法) system of measurements is a Japanese system with its origins in 13th century BC Chinese Shang dynasty. It spread from Japan in the 10th century BC and have relatives in South East Asia and Korea.
The name derives from the base unit of lenght shaku which equals 0.3030 metres. The length of a shaku has been almost unaltered since the 7th century. The system is only used in particular areas such as agriculture and carpentry since the metric system replaced the shakkan-hō in 1924.
Below, the name in Kanji is given after the names in the latin alphabet.
PostScript is a language for vector graphics created in 1982. It uses a fixed length system based on points which are exactly 1/72 of an inch.
TeX uses an slightly different point definition of 1/72.27 inch.