What is the calculation of an acre?

An acre was originally defined as the size of a plot of land that can be tilled by one man behind one ox in one day, and measures 66 feet by 660 feet, for a total of 43,560 square feet.[1]

In modern times, an "acre" is considered a square building lot of size 200 x 200 feet, however, this is really 8% shy of an acre. A real acre square lot would measure 208' 81/2" square.