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The square meter is the SI-derived unit of area. It has a symbol m² (33A1 in Unicode). It is defined as the area of a square whose sides measure exactly one metre. The square metre is derived from the SI base unit of the metre, which in turn is defined as the length of the path travelled by light in absolute vacuum during a time interval of 1⁄299,792,458 of a second.
Adding SI prefixes creates multiples and submultiples. However, as the unit is squared, the order of magnitude difference between units doubles from their comparable linear units. For example, a kilometre is one thousand times the length of a metre, but a square kilometre is one million times the area of a square metre.
A "square metre" is not the same thing as a "metre square"  - although it is true that exactly 1 square metre is exactly 1 meter square. But that only works for the numbers 1 and 0; no others.
For example, a square that is 2 metres long and 2 metres wide has 4 square metres of area.
2m * 2m = 4m2
But a square that is 4 metres squared would have 4 metres on each side. This means it would have 16 square metres of area.
4m * 4m = 16m2
That is, (4m)2 is 4 meters squared, whereas 4m2 is 4 square meters.
SI prefixes applied to the square metre
The square metre may be used with all SI prefixes used with the metre.
|100||square metre (centiare)||m²||100||square metre (centiare)||m²|
|102||square decametre (are)||dam²||10−2||square decimetre||dm²|
|104||square hectometre (hectare)||hm²||10−4||square centimetre||cm²|
|106||square kilometre||km²||10−6||square millimetre||mm²|
|1012||square megametre||Mm²||10−12||square micrometre||µm²|
|1018||square gigametre||Gm²||10−18||square nanometre||nm²|
|1024||square terametre||Tm²||10−24||square picometre||pm²|
|1030||square petametre||Pm²||10−30||square femtometre||fm²|
|1036||square exametre||Em²||10−36||square attometre||am²|
|1042||square zettametre||Zm²||10−42||square zeptometre||zm²|
|1048||square yottametre||Ym²||10−48||square yoctometre||ym²|
A square metre is equal to:
- "Unicode Utilities: Character Properties". http://unicode.org/cldr/utility/character.jsp?a=33A1.
- mathforum.org Miles squared vs. square miles.