Acres

Introduction
The acre is a unit of area in a number of different systems, including the imperial and U.S. customary systems. The most commonly used acres today are the international acre and, in the United States, the survey acre. One international acre is equal to 4,046.856 422 4 m2 exactly. One U.S. survey acre is equal to 62,726,400,000⁄15,499,969 m2 = 4,046.872 609 874 252 m2 approximately.
Conversions:
Acres to Hectares: 1 acre = 0.404685 hectares
Acres to Square Meters : 1 acre = 4046.856 m²
Acres to Square Feet : 1 acre = 43560 ft²
Acres to Square Kilometers: 1 acre = 0.004046 km²
Acres to Square Millimeters: 1 acre = 4046856422.4 mm²
Acres to Square Miles: 1 acre = 0.001562 mi²
Acres to Square Yards: 1 acre = 4840 yd²
Acres to Square Centimeters: 1 acre = 40468564.224 cm²
Acres to Square Inches: 1 acre = 6272640 in²
Acres to Square Micrometers: 1 acre = 4.046e+15 µm²
Acres to Square Microns: 1 acre = 4.046e+15 µm²
http://www.metric-conversions.org
Examples of what acres are used
One acre comprises 4,840 square yards or 43,560 square feet[1] (which can be easily remembered as 44,000 square feet, less 1%; or as the product of 66 x 660). Because of alternative definitions of a yard or a foot, the exact size of an acre also varies slightly. Originally, an acre was understood as a selion of land sized at one furlong (660 ft) long and one chain (66 ft) wide; this may have also been understood as an approximation of the amount of land an ox could plough in one day. A square enclosing one acre is approximately 208 feet and 9 inches (63.6 meters) on a side. But as a unit of measure an acre has no prescribed configuration; any perimeter enclosing 43,560 square feet is an acre in size.
Usefulness:
The acre is often used to express areas of land. In the metric system, the hectare is commonly used for the same purpose. An acre is approximately 40% of a hectare.
E.g.
One acre is 90.75 percent of a 53.33-yard-wide American football field. The full field, including the end zones, covers approximately 1.32 acres (0.53 ha).

Origin:
The word "acre" is derived from Old English æcer (originally meaning "open field", cognate to west coast Norwegian language "ækre" and Swedish "åker", German Acker, Latin ager and Greek αγρος (agros).The acre was approximately the amount of land tillable by one man behind an ox in one day. This explains one definition as the area of a rectangle with sides of length one chain and one furlong. A long narrow strip of land is more efficient to plough than a square plot, since the plough does not have to be turned so often. The word "furlong" itself derives from the fact that it is one furrow long.Before the enactment of the metric system, many countries in Europe used their own official acres. These were differently sized in different countries, for instance, the historical French acre was 4,221 square meters, whereas in Germany as many variants of "acre" existed as there were German states.