There are exactly 43,560 square feet in one acre. The acre is a unit of land area in the US customary and British imperial systems, traditionally defined as one chain by one furlong, which also equals 4,840 square yards or 1/640 of a square mile.
In metric terms, an acre is about 4,047 square meters, or roughly 40 percent of a hectare. A square plot of one acre would measure about 208.7 feet on each side.
Source: Wikipedia, Acre