Estimate how many gallons of paint you need from your wall area, the number of coats, and any openings.
gallons = paintable_area ÷ 350 × coats (round up)
One gallon covers roughly 350–400 ft² in a single coat. We use the conservative 350 ft² figure so you don't run short.
Most jobs need two coats. Treat primer as an additional coat at the same coverage.
Walls: perimeter 2 x (12 + 14) = 52 ft, times 8 ft = 416 square feet. Subtract a door and two windows, roughly 50 square feet, for 366 square feet of paintable wall. At 350 square feet per gallon and two coats: 366 ÷ 350 x 2 = 2.1 gallons, rounded up to 3 gallons.
If the walls are a dramatic color change or you're covering a dark wall with a light one, add a primer coat — treat it as a third coat at the same coverage, which pushes you to 4 gallons. Textured walls also drink more paint, so drop your coverage estimate to 300 square feet per gallon.
Coverage is the conservative end of the manufacturer label range (350 ft²/gal). Textured or porous surfaces use more.
Paint quantity scales with the number of coats and how thirsty the surface is. New drywall, bare wood, and patched areas drink the first coat, so they need primer or an extra finish coat. A typical gallon covers about 350 square feet per coat on a sealed surface, less on porous or textured walls — factor the real coat count, not just one pass.
Estimate primer separately for new or stained surfaces, and remember trim, doors, and ceilings are their own areas with their own coats. Trim paint covers less ground per gallon because it goes on in thin, careful passes. Deduct only large openings from wall area; small ones are cut around and covered by the buffer.
About 3 gallons for two coats at 350 ft² per gallon.
Divide the paintable area by about 350 square feet per gallon, then multiply by the number of coats. A 12-by-15-foot room with 9-foot walls needs roughly 2 gallons for two coats.
Prime new drywall, bare wood, patched repairs, and any drastic color change. Primer seals porous surfaces so the finish coats cover evenly and you use less topcoat overall.
Estimate trim and doors separately from walls. Trim paint covers fewer square feet per gallon because it is applied in thin, controlled passes, so budget extra relative to the linear footage.