How to Calculate the Number of Gallons of Paint

Primer and paint are sold according to how many square feet each manufacturer claims a gallon of their product will cover---usually around 350 to 400 square feet per gallon. The coverage will vary depending on the type of paint and its sheen, the attention given to preparing and priming the surface, the texture of the surface, environmental conditions and how many coats are required to complete the job. A pro paint job usually involves a primer when making dramatic color changes and painting over any dull, worn or new surfaces, and whenever shifting between oil-based and water-based paints. Most pros apply two finish coats over the primer.

Learning to estimate paint quantities will save you money and prevent waste
  1. Measure the dimensions of the surfaces to be painted and determine their square footage. A simple way to do this for a room with a uniform ceiling height is to measure the length of each wall, add the lengths together and multiply the sum by the height of the ceiling. For example, a square 10 foot by 10 foot room with a 10 foot high ceiling would be 40 feet times 10, or 400 square feet for the walls with another 100 square feet for the ceiling. The same concept works for exteriors, as well, but they may have to be broken into rectangles if roof heights are different on different parts of the house.

  2. Add all the square footages to be painted with the same color and paint type together.

  3. Subtract window and door openings from wall square footage as they will be painted with a different material or not at all. However, most pros do not subtract window and door openings, figuring the extra paint will account for loss in rinsing out brushes, pouring between containers and overall loss in roller covers. The extra paint will also be needed for touch ups.

  4. Obtain the coverage rate for the primer and paint of choice from the paint store or paint manufacturer's website.

  5. Divide the number of square feet to be painted by the coverage rate of the paint to determine the number of gallons needed.

  6. Multiply the number of gallons by the number of coats that will be needed for the job to determine the total amount of paint to purchase.

  7. Adapt the quantity of paint to be purchased to take advantage of possible savings by buying in different size containers. Custom tinted paint comes in quarts, gallons and five-gallon buckets. There is often a price break for buying in larger size containers. Aim not to have excess amounts of paint leftover.

About the Author

Steven Sester has written and published for others as a public relations professional since the 1970s. His areas of expertise include the fine and performing arts, home improvement, emerging technology, alternative healthcare, environmental and sustainability issues, entrepreneurship and a variety of other topics. He is a graduate of the New College program at San Jose State University.