Each exterior and interior house paint lists the total squarefoot coverage in the information on the paint can. Normal coverage is between 300 to 400 square foot per gallon. It is important to know the square foot coverage to calculate how many gallons of paint are required. The same measurement process is used to calculate paint for the interior rooms of a home or building. Determine the amount of paint necessary to paint the exterior of a home using exterior measurements and math computation. Determine the size of the windows and doors to avoid buying too much paint.
Things You'll Need
 Tape measure
 Paper
 Pencil
 Ladder
 Calculator

Draw a rough diagram of the house. Measure the width and height of each wall and write it down. Multiply the width and the height of each wall to determine the square footage. Write the answer down. Add the squarefootage numbers together to attain the total square footage.

Draw a second diagram of the house to use for windows and doors. Measure the height and width of the windows and doors on each side of the house. Write the dimensions down next to the appropriate side wall. Multiply the width and the height of each door and window to determine their square footage. Write all answers down. Add the square footage numbers from each window and door to attain the total square footage.

Subtract the total square footage of the windows and doors from the total square footage. Write the number down. This is the total surface area that requires paint.

Multiply the total surface area that requires paint by 10 percent. This extra allows for touchups, repairs and spillage.

Divide the total surface area that requires paint plus 10 percent by the squarefoot coverage area listed on the paint can. The answer is how many gallons of paint are necessary.
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References
 Photo Credit tape measure image by Joann Cooper from Fotolia.com