Area

Area is a quantity expressing the size of a region of space. Surface area refers to the summation of the areas of the exposed sides of an object.

Units

Units for measuring surface area include:

square metre = SI derived unit
are = 100 square metres
hectare = 10,000 square metres
square kilometre = 1,000,000 square metres
square megametre = 1012 square metres

Imperial units, as currently defined from the metre:

square foot (plural square feet) = 0.09290304 square metres.
square yard = 9 square feet = 0.83612736 square metres
square perch = 30.25 square yards = 25.2928526 square metres
acre = 160 square perches or 43,560 square feet = 4046.8564224 square metres
square mile = 640 acres = 2.5899881103 square kilometres

The article Orders of magnitude links to lists of objects of comparable surface area.

Useful formulae

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