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How much paint is needed to paint a room depends on two quantities. First, the surface area in square feet to be covered needs to be calculated. Secondly, the surface area covered by a gallon of paint needs to be found in the instructions on the paint can. This first quantity is divided by the second to determine the number of gallons of paint is required, which is then rounded to the next higher gallon or quart.

# Calculate Area of Walls and Ceiling

Measure the height and width of each wall in feet. Multiply these together to find the area of the wall. Add the areas of all the walls together. Find the area of the ceiling by finding the area of the floor. This, of course, assumes that all walls and the ceiling are perfect squares or rectangles. For slanted or sloped ceilings, all the edges of each wall should be measured. Calculating the areas of odd shaped walls may require more geometry knowledge. For example, if one upper corner of a wall is higher than the other upper corner, an average of the two vertical edges would have to be taken before multiplying by the width of the wall to find the area. A shortcut to the total room measurement is possible when all the walls and ceiling are perpendicular to each other. Measure the height, width, and length of the room. Then add the length and the width, double this sum, and multiply by the height to get the area of the four walls. To add the ceiling area, multiply the length and the width and add this product to the wall area. For example, a room 10 feet high by 9 feet wide by 12 feet long would have a surface area of 9 + 12 = 21, 21 doubled is 42, and 42 x 10 = 420 square feet for the four walls. The area of the ceiling would be 9 x 12 = 108 square feet, and if this is added to the walls, the sum is 420 + 108 = 528 square feet.

# Subtract Area of Doors and Windows

The height and width of each door and window is measured in feet and these two measurements are multiplied together to find its area. Whether to measure the inside or outside of the doorframe depends on whether this trim will be painted along with the rest of the room. If not, then the outside edges of the door trim should be measured in feet. Add up the areas of all the doors and windows in the room in square feet together, and subtract it from the total area of the walls and ceiling. For example, if a room has one door measuring 3 feet in width by 6 feet in height, its area is 3 x 6 = 18 square feet. If it has one window 4 feet high by 3 feet in width, 4 x 3 =12 square feet. Adding these together gives 18 + 12 = 30 square feet of area not covered by the particular paint, and when subtracted from the gross area of 528 square feet, gives a net area of 528 – 30 = 498 square feet.

# Multiply by Number of Coats

The calculated area of the surface area to be painted is multiplied by the number of coats of a particular paint or primer is to be applied to that surface area. A separate calculation is used for paint and for primer. For example, if one coat of primer and two coats of paint, multiply the area by one for the primer and double it for the paint itself. For example, our example room would require coverage of 498 x 1 = 498 square feet to be covered by one coat of primer and 498 x 2 = 996 square feet to be covered by two coats of paint.

# Divide by Area Covered by One Can of Paint

The instructions on the paint can should include the amount of surface area in square feet that are covered by a can of paint. Instead of this figure, the amount given might be for another unit than the amount of paint in one can. For example, the instructions may say that the can of paint will cover 300-400 square feet. This is fine if the can contains a gallon of paint. But if the same paint also comes in quart-sized cans, four quarts will equal one gallon, and the units of this information should be checked to see if it is in square feet per can or square feet per gallon. Divide the area of surface to be painted by the number of square feet covered by a gallon of paint to see how many gallons are needed to cover the area. If this number is less than or equal to 0.75, multiply by four for the number of quarts, rounding up to the next whole quart. If the number of gallons is greater than 0.75 but less than 1, buy a whole gallon. If more than one gallon, then get that number of gallons, and round the fraction to the next higher quart or gallon. For example, say a gallon can of paint can cover a surface area of 300-400 square feet. Dividing the doubled net area of our room of 996 square feet by the coverage per gallon of the paint yields 996 / 300 = 2.23 gallons to 996 / 400 = 2.49 gallons. Rounding up to the nearest quart yields 2 gallons plus 1 or 2 quarts of paint to put two coats of the paint on the walls and ceiling of our example room. The primer, based on 528 square feet, would yield half that amount, about 1 gallon plus 1 quart.

Resources

“Paint Calculator – How Much Paint Do I Need to Buy for My Room?.” Benjamin Moore & Co.. N.p., n.d. Web. 13 June 2011. http://www.benjaminmoore.com/bmpsweb/portals/bmps.portal?_nfpb=true&_windowLabel=contentrenderer_1_4&contentrenderer_1_4_actionOverride=%2Fbm%2Fcms%2FContentRenderer%2FrenderContent&contentrenderer_1_4currentNodeUUID=%2FBEA+Repository%2F6002&contentrender.

“Estimate How Much Paint For a Job or Project.” Sherwin-Williams. N.p., n.d. Web. 13 June 2011. http://www.sherwin-williams.com/do_it_yourself/painting_techniques/paint_calculator/