Acre

Origin:
The word "acre" is derived from Old English æcer originally meaning "open field", cognate to west coast Norwegian 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. Historically, the size of farms and landed estates in the United Kingdom was usually expressed in acres (or acres, roods, and perches), even if the number of acres was so large that it might conveniently have been expressed in square miles. For example, a certain landowner might have been said to own 32,000 acres of land, not 50 square miles of land.
Relation to SI unit:
1 international acre is equal to the following metric units:
• 4,046.8564224 square metres
• 0.40468564224 hectare (A square with 100 m sides has an area of 1 hectare.)
1 United States survey acre is equal to:
• 4,046.87261 square metres
• 0.404687261 hectare
1 acre (both variants) is equal to the following customary units:
• 66 feet × 660 feet (43,560 square feet)
• 1 chain × 10 chains (1 chain = 66 feet = 22 yards = 4 rods = 100 links)
• 1 acre is approximately 208.71 feet × 208.71 feet (a square)
• 4,840 square yards
• 160 perches. A perch is equal to a square rod (1 square rod is 0.00625 acre)
• 10 square chains
• 4 roods
• A chain by a furlong (chain 22 yards, furlong 220 yards)
• 1/640 (0.0015625) square mile (1 square mile is equal to 640 acres)
Disadvantages over other units:
It would be difficult to calculate the exact amount of acre as it has a lot of decimal points hence I feel that using square metres would be easier to use compared to acre.