Units of measurement
A unit of measurement is an amount used to show how much length, or mass, or volume or other quantities something has. There are many different standards and units used all over the world. One global standard is commonly called the Metric System. It is also known as Système International d'Unités, or SI. The kilometre, metre, centimetre, millimetre are some units of length or linear size in this system. While many countries have officially adopted SI, a wide range of traditional units are still used. For example, in the United States the metric system has been legal for trade since 1866 and is widely used in industry and daily life. Some basic measurements such as the gallon, inch, mile, and the pound are still widely used. See US units of measurement. In Europe, which is largely metric, this system based on tens is widely rearranged into odd, but practical human divisions. For example, the quarter millimetre, and non-power of ten trade quantities which are marked with the EEC or 'e' mark.
- The unit of volume is the metric litre which is used for water, petrol and other liquid. A litre is equal to a 10cm by 10cm by 10 cm (or decimetre cube. There are approximately 3.78541 litres in one US gallon. A millilitre is a 1cm by 1cm cube - this is also called ml or cubic centimetre or one CC. Cooks often use this measurement. A typical teaspoon is about 5 ml in size.
- The unit of mass is the metric gram which; on Earth is quite light. On Earth it weighs the same as a 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 10x10x10=1000. The metric tonne is 1000 kilograms, a million grams.
- The unit of time is the second. 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 metric but are used in finance and other industries that set some standards.
The imperial units now used mainly in the United States: 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 that is 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.
A unit of measurement that applies to money is called a unit of account. This is normally a currency issued by a country - and smaller divisions. For instance the US dollar and US cent, which is 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 more in these fields.