1. Multiply the dimensions of the fence to calculate its area in square feet. Area = 20 x 1.5 = 30 square feet. 2. Obtain the plank width (typically in inches) from specifications
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A linear foot is a unit of length. A square foot is a unit of area. The two units are therefore incompatible.
http://wiki.answers.com/Q/How_many_linear_foot_in_...

I've never heard them used together but if it helps I'll tell you what I know about cabinet pricing. I was priced custom cabinets @ 85 dollars a linear foot. I have 32 feet of wall.
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A square foot is a unit of measurement of area. A piece of material measuring 12 inches by 12 inches is one square foot. To compute the number of square feet in a piece of wire mesh
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