Insulating your home provides a barrier not only against the heat and cold of the outside, but helps you keep heating and air conditioning inside as well. Insulation can be installed through the use of fiberglass batts or blown insulation that fills the cavities between your wall studs, sealing the gaps where air can transfer between the outside and inside of your home. Most people assume that it's complicated to calculate the needed insulation due to taking into account wall studs and windows, but it's not. The process to calculate needed insulation is easy because insulation can compress between the studs.
Things You'll Need
 Tape measure
 Calculator
 Pen and paper

Find the area in square feet of the wall you wish to insulate. Measure the height and width and multiply them together. For example, if your wall is 8 feet high by 14 feet long, then the total wall area is 8 x 14 = 112 square feet.

Find the total area of all the openings in that wall. Measure the height and width of each door and window in the wall, compute the area of each and then add them together. If you have one window (3 x 2) and one door (6.5 x 2.5), then the area of the window is 3 x 2 = 6 square feet and the area of the door is 6.5 x 2.5 = 16.25 square feet. Add the areas together and your total opening area is 6 + 16.25 = 22.25 square feet.

Subtract the total opening area from the total wall area to find the area requiring insulation. In this example we have 112  22.25 = 89.75. You need enough insulation to cover 89.75 square feet.

Purchase your insulation. If you plan on using blown insulation, read the label on each bag to determine the squarefoot coverage of the bag and purchase enough to equal the total squarefoot coverage that you need. If you are purchasing fiberglass batt insulation, measure between your wall studs to determine if you need to purchase the 15 or 23inch batt (which ever fits between your wall studs). Read the label on the batt to find out how many square feet it covers and purchase enough rolls to equal the squarefoot area that you need to insulate.
Tips & Warnings
 Check out the chart listed in the Resource to ensure that you have chosen insulation with the right R value for the area where you live. The R value is the measurement of the resistance of the insulation to cold and heat.
 Don't leave gaps in your insulation. Any gap will allow air flow into and out of the home, diminishing the effectiveness of the insulation you have installed.
References
 Photo Credit spiraling insulation image by jbattx from Fotolia.com