The acre was a bit of a movable feast originally, being the area of land that a early medieval European ploughman with two oxen could work in an average day in the open field system. That might depend a fair bit upon soil type or the year's weather.
When it needed standardising it became 66 feet (20.1m) by 660 feet (201m), 43500 square-feet (4050m^2) in England. That's 1/80 of a mile by 1/8 of a mile which was the fashion at the time, chains and furlongs and such.