How was the measurement of an acre...

In the British Imperial and U.S. Customary Systems, an acre was originally a unit of land equal to 43,560 square feet, or 160 square rods. The word is derived from the Latin word ager, meaning “field.” The basis for choosing those measurements came from a primitive time when a field or plot was designated by the oxen needed to plow it or the seed needed to sow it. Thus, the Anglo-Saxons defined an acre as a strip of land 1 1/10 furlong, or 40x4 rods (660x66 feet).