# Units of measurement

Disparate systems of units of measurement used to be very common. Now there is a global standard, the Systeme International (SI) system of units (informally known as the Metric System), which has been or is being adopted in most major countries of the world (except the U.S., which is described in more detail in Metric system in the United States). Except in the United States, many will know how to translate from these units. For more detail on conversion, see US customary units.

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## SI units

The SI units are defined and agreed internationally.

• The unit of mass is the kilogram (kg). On Earth one gram (10-3 kg weighs the same as 1 millilitre of water at exactly 0 degrees Celsius, a bit too warm to turn into ice. A more common measure is the kilogram. This weighs the same as a litre of water - exactly 1000 grams because 10 × 10 × 10 = 1000. The tonne is 1000 kilograms, or a million grams.
• The unit of time is the second (s). The minute (60 seconds) and hour (60 minutes or 3600 seconds) are larger units. The day is usually said to be 24 hours, but is actually a tiny bit longer. This difference is corrected at the end of every year. A week (7 days) and month are also standards in most places, but there are different calendars. These are not part of the SI system, but are used in finance and other industries that set some standards.

Standards are very important. Each unit is a set size. A distance or length or volume or mass or span of time being measured is described as a certain number of these units. Sometimes more than one unit is used to get better accuracy, or a smaller unit is used - so one can say "one metre, 95 centimetres" or just "195 centimetres".

## Imperial or US Customary units

The imperial units were mostly used in the United Kingdom and related countries. Some are still used but metric units are used almost always now and are much more common. The imperial units are now used mainly in the United States, where they are known as US customary units: inch, foot, yard, mile, and the US gallon, smaller than the old imperial gallon (UK Gallon), about 4.5 litres, which is now not used anywhere much. Some older bottles, jugs and liquid containers are this size. There are 128 fluid ounces in a US gallon, but there are also 16 ounces in a pound, an older measure of weight (about 453.6 grams). These are two different measurements with exactly the same name - it is only when measuring water that they actually mean the same.

## Natural units

The above systems of units are based on arbitrary unit values, formalised as standards. Some unit values occur naturally in Science. Systems of units based on these are called Natural units, for example Planck units.

## Other units

A unit of measurement that applies to money is called a unit of account. This is normally a currency issued by a country or a fraction thereof; for instance, the US dollar and US cent (1/100 of a dollar), or the Euro and Eurocent.

Science, medicine and engineering use larger and smaller units of measurement than these, and talk about them more exactly. For instance, the difference between mass and weight matters a lot more in these fields. In these fields, the judicious selection of the units of measure can actually aid the researchers in both framing and solving the problem.

• Art and Cultures
• Countries of the World (http://www.academickids.com/encyclopedia/index.php/Countries)
• Space and Astronomy