How Much Paint for a Room?
Paint cans list coverage in square feet per gallon, but that number assumes a smooth, sealed surface and a single coat on a blank wall. Real rooms have doors, windows, texture, and often two coats. This guide walks through perimeter-based wall area, sensible opening deductions, and how coverage and coat count combine into a shopping list you can trust.
Use the Paint Calculator →Steps
- Measure perimeter and height — Add all wall lengths; multiply by ceiling height for gross wall area.
- Subtract openings — Roughly 21 ft² per door and 15 ft² per window — or measure exactly.
- Apply coats and coverage — Gallons = paintable ft² × coats ÷ coverage (often 350–400 ft²/gal). Round up.
Wall area from perimeter
Gross wall area = room perimeter (ft) × ceiling height (ft). For a 12×12 room with 8 ft ceilings, perimeter is 48 ft and gross walls are 384 ft².
Open floor plans: sum each enclosed wall segment that will receive paint. Ceilings are calculated separately if you are painting them — length × width for a flat ceiling.
Door and window deductions
Standard interior door: about 21 ft² (3 ft × 7 ft). Small window: about 15 ft². These defaults are editable in our calculator if your openings differ.
Do not deduct baseboard-height slivers — the roller still loads paint into those zones. Over-deducting openings is a common reason people run short on the last wall.
Example walkthrough
12×12 room, 8 ft ceilings, one door, one window: 384 − 21 − 15 ≈ 348 ft² paintable. Two coats at 350 ft²/gal → 348 × 2 ÷ 350 ≈ 1.99 gal → buy 2 gallons.
Coverage and coats
Premium interior latex often covers 350–400 ft²/gal on a primed, similar-color wall. Deep color changes, unprimed drywall, or heavy texture may drop effective coverage to 250–300 ft²/gal.
Two coats is standard for uniform color and durability. One coat may suffice when refreshing the same hue on clean walls — check the can and your contractor's spec.
Practical buying tips
Round up to the next whole gallon on multi-room jobs so sheen and batch match. Borderline calculations (1.8 gal) deserve an extra quart rather than a return trip.
Keep the formula label — if the store mixes custom color, you may need the same formula for touch-ups later.
Frequently asked questions
- Does primer change the math?
- Use the same area formula with primer coverage from the can — often one coat.