Insulation comes packaged in many different sizes and each package has the number of square feet marked on it. You must know the total square feet of an area to be insulated and the number of bags required to determine the price per square foot. A construction calculator makes this an easy calculation, but if you don't have one, a regular calculator will do.
Things You'll Need
 Calculator

Multiply the length by the width (or height) of an area to be insulated. For example, if the length of walls to be insulated added up to 40 feet 8 inches and the ceiling was 8 feet high, the area would be 325.36 square feet (40.67 x 8 = 325.36). The 8 inches is converted to a decimal number by dividing it by 12.

Subtract the square footage of windows, doors or other openings from the area to be insulated. Calculate the square feet by multiplying the width by the height (or length) of each opening.

Determine the number of bags of insulation needed by dividing the total square feet to be insulated by the number of square feet in each bag.

Multiply the number of bags needed by the cost per bag including taxes and shipping to determine the total cost.

Divide the total cost by the total square feet to calculate the price per square foot. For example, if the total cost was $90 for the insulation and you had 275 square feet to insulate, the price per square foot of insulation would be about 33 cents per square foot.
Tips & Warnings
 Walls, floors and ceilings require different thicknesses of insulation. Each area should be calculated separately and then added to the totals.