The metric system (1791- ) is the system of weights and measures used by almost the entire world. Everyone learns it in school, even the Americans, who still use English weights and measures.
The metric system has dozens of units of measure, most of them used only in science. Here are the units that come up most often in everyday life:
- second (s) for time. There are 60 seconds in a minute, 60 minutes in an hour and 24 hours in a day. That makes 86,400 seconds in a day. Most people speak at a rate of two or three words a second.
- metre (m) for length. By definition, light goes 300 million metres a second. My father was almost two metres, which is tall.
- litre (L) for an amount of space. A thousand litres are in a space that is one metre long, one metre high and one metre deep. Most bottles hold about a half litre, a cup a quarter litre.
- gram (g) for mass or weight. A litre of water has 1000 grams. An apple has a weight of about 115 grams, a small copper coin is a few grams.
- Celsius (C) for how warm it is. Water becomes ice at 0 C and boils away at 100 C. If it gets much below 15, you will want to put on a coat before you go outside.
- are (a) for land area. An are is an area that is ten metres wide and ten metres long or 100 square metres. No one uses are by itself, but hectares (100 ares) instead – an area that is 100 metres by 100 metres. A football field is about three-quarters of a hectare; an American football field is half of a hectare.
Americans write metre as “meter” and litre as “liter”, just as they write theatre as “theater”.
The metric system uses prefixes to make these units larger or smaller. Here are the most commonly used prefixes:
- kilo (k): 1000
- hecto (h): 100
- centi (c): 1/100th
- milli (m): 1/1000th
So a kilometre (kilo + metre) is 1000 metres. A milligram (milli + gram) is a thousandth of a gram. And so on.
Just from the most common bits that I have presented so far, we have 30 possible units, but in practice only these are used:
- second (s)
- millisecond (ms)
- kilometre (km)
- metre (m)
- centimetre (cm)
- millimetre (mm)
- litre (l)
- millilitre (ml), also called cc for cubic centimetre
- kilogram (kg), also called a kilo
- gram (g)
- milligram (mg)
- Celsius (C), sometimes called centigrade
- hectare (ha)
A thousand kilograms is also known as a metric ton.
The “kg” and “mm” and so on are used when you do not want to write out the whole name: “I am 1.8 m tall.”
Years, days, hours and minutes are not part of the metric system, but people still use them freely.
The metric system was invented by the French in the late 1700s soon after they had overthrown their king and were remaking the world along more rational lines. It was designed to be simpler than any existing system.
See also:
Abagond 
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