How Much Paint Do I Need? A Room-by-Room Guide

The short answer

One US gallon of interior paint covers roughly 350 square feet in a single coat on smooth, primed drywall. Most walls need two coats, so a good rule of thumb is: paintable area × 2 ÷ 350, rounded up to whole gallons.

For an average 12 × 14 ft room with 8 ft ceilings — walls only, two coats, one door and two windows subtracted — that's about 2 gallons. Add the ceiling and you're closer to 3.

Try it Paint Calculator → Estimate how many gallons of paint a room needs, with doors and windows subtracted and the number of coats included.

Step 1 — Measure the walls

Wall area is the room's perimeter times the ceiling height: 2 × (width + length) × height. A 12 × 14 room with 8 ft ceilings is 2 × 26 × 8 = 416 sq ft before any deductions.

• Measure to the nearest half-foot; don't trust the floor plan.
• Sloped or vaulted ceilings: average the height, or split the wall into a rectangle plus a triangle.
• Painting the ceiling too? Add width × length.

Step 2 — Subtract doors and windows

Openings don't get painted, so subtract them or you'll over-buy. Use about 21 sq ft per standard door and 15 sq ft per window. Our example loses 21 + (2 × 15) = 51 sq ft, leaving 365 sq ft of paintable wall.

Step 3 — Coats, coverage and primer

Multiply paintable area by the number of coats, then divide by the coverage on your can (usually 300–400 sq ft/gallon). Two coats over 365 sq ft is 730 sq ft ÷ 350 ≈ 2.1, so buy 3 gallons and keep the extra for touch-ups.

• Use primer over bare drywall, big color changes, stains or repairs — it's separate from your topcoat gallons.
• Rough, porous or dark walls drink more paint; drop coverage toward 250–300.
• Buy all cans in one trip so they're from the same batch — mixed batches can leave a seam.