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 square-footage 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 touch-ups, repairs and spillage.
Divide the total surface area that requires paint plus 10 percent by the square-foot coverage area listed on the paint can. The answer is how many gallons of paint are necessary.