How to Calculate How Much Paint You Need for a Room
Calculating paint quantity requires measuring wall area, accounting for doors and windows, choosing the right coverage rate, and applying a waste factor. This guide walks through every step for walls, ceilings, and trim.
Step 1 – Measure the Wall Area
Start by measuring the perimeter of the room. Walk around and measure the length of each wall individually, then add them together to get the total perimeter. For a 12-by-16-foot room, the perimeter is 12 plus 16 plus 12 plus 16, which equals 56 feet.
Multiply the perimeter by the ceiling height to get the total wall area. For an 8-foot ceiling: 56 times 8 equals 448 square feet. This is the gross wall area before subtracting openings.
For rooms with angled walls, niches, or built-in bookshelves that will be painted a different color, measure those sections separately and exclude them from the main wall calculation. Treat each distinctly painted surface as its own area.
Step 2 – Subtract Doors and Windows
Subtract the area of each door and window opening from the gross wall area. A standard interior door (36 inches by 80 inches) has an area of approximately 20 square feet. A standard double-hung window (36 inches by 48 inches) has an area of approximately 12 square feet. Use the actual dimensions of your doors and windows for greater accuracy.
For each door, multiply its width in feet by its height in feet. For each window, do the same. Add up all the door and window areas and subtract the total from the gross wall area. For the 448-square-foot example room with two doors (20 square feet each) and two windows (12 square feet each), subtract 20 plus 20 plus 12 plus 12, or 64 square feet. Net wall area: 384 square feet.
If the door and window trim will be painted a different color (as is common when using a semi-gloss trim paint), do not subtract the trim area separately at this step. Calculate the trim as its own surface area and handle it with a different paint quantity calculation.
Step 3 – Add the Ceiling Area
If the ceiling will be painted, calculate its area separately. For a rectangular room, the ceiling area is simply the room length times the room width. For the 12-by-16-foot room, the ceiling area is 192 square feet.
Vaulted or cathedral ceilings require calculating the sloped surface area rather than the horizontal floor area. Measure the horizontal width and the slant length of each ceiling slope, multiply them together, and add the resulting areas for all slopes. This is the same calculation used for roof area.
Ceiling paint is typically applied in a single coat when using a dedicated ceiling paint product, since white ceiling paint covers well in one pass. If you are painting a vaulted ceiling in a deep color or over a dramatically different existing color, plan for two coats and calculate accordingly.
Step 4 – Find the Coverage Rate and Number of Coats
Check the coverage rate printed on the paint can. Most interior latex paints cover 350 to 400 square feet per gallon. Use the lower end of the range for textured surfaces and the upper end for smooth, primed walls in similar colors.
Determine how many coats you need. One coat is sufficient for same-color touch-ups and refreshes. Two coats are needed for new construction, drastic color changes (especially going from dark to light), and any surface that has not been previously painted. Three coats are sometimes required when going from a very dark color to white or a light pastel.
For each coat, divide the paintable area by the coverage rate to find gallons per coat. For 384 square feet of wall area at 350 square feet per gallon: 384 divided by 350 equals 1.10 gallons per coat. For two coats, multiply by 2: 2.20 gallons. Add 10 percent for waste: 2.20 times 1.10 equals 2.42 gallons. Purchase 3 gallons (2 gallons plus 1 quart is insufficient; buy 3 gallons to be safe).
Step 5 – Calculate Trim and Ceiling Separately
Calculate the trim paint quantity by estimating the total linear feet of trim (baseboards, door casings, window casings, crown molding) and multiplying by the average trim width in feet. Standard 3.5-inch baseboard is 0.29 feet wide. For 80 linear feet of baseboard: 80 times 0.29 equals 23.2 square feet.
Trim paint is typically applied in two coats because trim surfaces receive more wear and the semi-gloss finish reveals surface imperfections more readily than flat wall paint. At 400 square feet per gallon for semi-gloss trim paint, 23.2 square feet of trim requires less than 0.06 gallons per coat. One quart of trim paint is more than enough for most single-room trim jobs.
Keep ceiling, wall, and trim paint purchases separate even when they share the same base color. Different sheens (flat for ceilings, eggshell for walls, semi-gloss for trim) look best on different surfaces and perform differently under daily conditions. Using ceiling flat paint on walls, for example, results in a surface that marks and scuffs easily and is difficult to clean.