What Exactly is an Acre?
In all English-speaking countries, land is traditionally measured by the acre, a very old Saxon unit that is either historic or archaic, depending on your point of view. There are references to the acre at least as early as the year 732 AD. The word “acre” also meant “field”, and as a unit an acre was originally a field of a size that a farmer could plow in a single day. In practice, this meant a field that could be plowed in a morning, since the oxen had to be rested in the afternoon. The French word for the unit is journal, which is derived from jour, meaning “day”; the corresponding unit in German is called the morgen (morning) or tagwerk (day’s work).
Most area units were eventually defined to be the area of a square having sides equal to some simple multiple of a distance unit, like the square yard. But the acre was never visualized as a square. An acre is the area of a long and narrow Anglo-Saxon farm field, one furlong (40 rods) in length but only 4 rods (1 chain) wide. This works out, very awkwardly indeed, to be exactly 43,560 square feet. If we line up 10 of these 4 x 40 standard acres side by side, we get 10 acres in a square furlong, and since the mile is 8 furlongs, there are exactly 640 acres in a square mile.