Metric measurements
Metric area measurements are based on the meter, with the main unit being a Hectare, 10000m2.There are exactly 640 acres in a square mile.
Imperial / American measurements
These area measurements are mostly square versions of their linear conterparts except for the acre which is an area of length 1 furlong and a width of 1 chain. The Old English word "acre" means field and it was generally considered the area that could be ploughed in a day using a yoke or oxen.