Paint Calculator
Estimate how many gallons of paint you need for a room from wall dimensions, doors, windows and number of coats.
How much paint do I need?
Find the wall area, subtract doors and windows, multiply by the number of coats and divide by the coverage on the can.
Gallons = (Perimeter × Height − Doors − Windows) × Coats ÷ Coverage
A 12 × 12 ft bedroom with 8 ft ceilings has 48 × 8 = 384 ft² of wall. Subtract one door (21 ft²) and two windows (30 ft²) for 333 ft². Two coats at 350 ft² per gallon need 1.9 gallons, so buy 2 gallons.
Coverage varies
Most interior paints cover 350–400 ft² per gallon on smooth, primed walls. Expect less on textured walls, bare drywall, brick or when making a big color change. Rough surfaces can drop coverage to 250 ft² per gallon or lower.
Do you need two coats?
Usually, yes. Two coats give an even color and better durability. One coat may do if you are repainting the same color in good condition. Going from dark to light often needs a primer plus two coats.
Buying tips
- Buy all the paint at once so the tint batch matches; mix cans together ("boxing") for large rooms.
- If you need just over a whole number of gallons, a quart (one-quarter gallon) may cover the gap.
- Keep a small amount for touch-ups, labelled with the room and color code.
- Ceilings usually take a flat ceiling paint; trim and doors are often semi-gloss and bought separately.
Frequently asked questions
How much paint do I need for a 12x12 room?
With 8 ft walls and two coats, about 2 gallons for the walls, plus about 1 gallon for the ceiling if you paint it.
How many square feet does a gallon of paint cover?
About 350–400 square feet per coat on smooth, primed surfaces.
Do I need primer?
Use primer on new drywall, bare wood, stains, glossy surfaces or when making a dramatic color change. Paint-and-primer products can work on previously painted walls.
Last reviewed Oct 20, 2026. Results are estimates; check important decisions with a qualified professional.