Price may be given in linear foot, per item or per square inch, and you have to convert it to price per square foot. For example, a three-inch-wide floorboard may be priced by how long it is, but you need to know how much it will cost to cover a whole floor---a floor that you already know the square footage of. Or you may need to shingle a roof that you know the square footage of, but the price is given per shingle.
To convert lineal feet
Determine the exposed width. For example, a floorboard that will have an exposed width of 3 inches would be converted to 1/4 feet. Use a tape measure if you need to.
Convert this width to feet. For example, the 3 inches (or 7.5cm) is 1/4 feet.
Divide the lineal price (price per length, in feet) by the number of feet the width is. For example, a floorboard that's £3 per lineal foot and 1/4 feet wide has a cost per surface area of £3/(1/4)=£20/foot-square.
To convert price per unit
Determine the exposed area of the unit. For example, a shingle may be 6 inches by 8 inches, but only 6 inches by 6 inches will be exposed when the shingles are layered on the roof.
Convert the measurements to feet and multiply. For example, the 6 inches by 8 inches becomes 1/2 feet times 3/4 feet, which is 3/8 feet squared.
Divide the price per unit by the exposed area. For example, if the price is 10p per shingle and the area is 3/8 square feet per shingle, the square feet price is .2 times 8/3, or 34 per square foot. Use a calculator, if you need to.
Note that you can convert price per square inch into price per square foot by just multiplying the price by 144. This is because there are 144 square inches in each square foot. 1 foot time 1 foot equals 12 inches times 12 inches, which in turn equals 144 square inches.
You can't convert from linear feet to square feet without knowing the width. In the example above, 3 inches was used. When doing your own calculating, just insert your width number where you saw 3 inches.