Good question. There are lots of factors involved, but for most interior and exterior projects the following will usually give you an accurate estimate of your needs. Simply calculate the square footage of the surfaces to be painted and divide by the number of square feet that the manufacturer indicates can be covered by a gallon of your selected paint.
For example, if a room is 10 feet by 20 feet with 8-foot ceilings, your square footage is 480 (the circumference times the ceiling height). If you have a number of windows and doors, subtract the square footage of those openings (standard doors are about 21 square feet, standard windows about 15 square feet). For many paints, including primers and stains, a gallon will cover about 400 square feet (the product label will show the coverage). Since two coats are normally recommended for most projects, two gallons would be needed for the 10-by-20-foot room example. (This assumes there are one or two windows.) Remember, it’s always a good idea to have a little leftover paint for future touchups.
As mentioned earlier, there are a number of factors that affect how much paint you’ll need. These include the type of surface being covered and the color currently on thesurface as well as the one being applied. The best way to ensure that you have the proper amount of paint for your project is to take your measurements and other information (surface being covered, its condition and color) to your local independent paint retailer.