QUICK ANSWER
How much paint do I need for a 10×12 room?
With 8-foot walls, one door, two average windows, and two coats, a practical estimate is about 2 gallons at roughly 350 sq ft of coverage per gallon.
Perimeter 44 ft × 8 ft = 352 sq ft before openings
Worked example
Subtracting roughly 50 square feet for one door and two windows leaves about 302 square feet per coat. Two coats equal about 604 coat-square-feet. At 350 square feet per gallon, the mathematical result is about 1.73 gallons, so rounding up gives 2 gallons.
Coverage varies
Texture, color change, primer, product formulation, and application method can all affect actual coverage.