Painting the interior or exterior of a home is an easy and relatively inexpensive way to update its look and personality, although figuring out the right amount of paint to purchase can be tricky for many people. Guessing often results in unused paint and wasted money, or in a trip back to the paint store to purchase an additional gallon. Using a few simple calculations, however, you can accurately measure the amount of paint needed, saving yourself both time and money.
Measure the length and width of the area you want to paint. Multiple the two measurements together to find the area's total square footage.
Measure the length and width of any areas that don't need paint, such as windows or doors. Multiply the measurements together to get the total non-painting area.
Subtract the square footage of the areas not to be painted from the overall paintable surface area. For example, a 10-foot-by-12-foot wall has a total surface area of 120 square feet. If the wall has a 3-foot-by-4-foot window (12 square feet), then the total area to be painted would decrease to 108 square feet.
Visit a paint store and determine the type of paint you plan to purchase. On the can, locate the paint's spread rate--the number of square feet a gallon will cover in a single coat. Most paints have a spread rate of approximately 400 square feet per gallon.
Divide the area to be painted by the spread rate. For example, to the exterior of a home with 1,400 square feet of paintable surface, you will need approximately 3.5 gallons of paint for a single coat.
To achieve the best coverage, you may have to apply multiple coats of paint, especially when painting the exterior of a home. Keep this in mind when calculating how much paint you'll need for your project.
It's better to have too much paint than not enough. Buying all your paint at the same time will help ensure color consistency across different cans and it never hurts to have a little extra touch-up paint on hand.