Does an acre have to have a specific shape? In The Wall by Pink Floyd, the teacher says the area of an acre is a rectangle that is one furlong in length and one chain in width. How long is a chain and how wide is a furlong?
Not as such. The original definition of an acre was, indeed, a furlong by a chain, a definition used by surveyors to measure plots. However, the area one derives from that measurement is an acre, even if it's contained within a different shape.
1 furlong is equal to 10 chains, or 660 feet. If you do the math, this means an acre is 10 square chains, or 43,560 square feet. An oval-shaped 43,560 square-foot plot will still be 1 acre in size.
For more information, and metric equivalents, try our nifty conversion calculator.
—The Fact Monster