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.
About 3 gallons for two coats at 350 ft² per gallon.