How Much Paint for a Room?
Do not blindly trust the square foot coverage on a paint can. That number assumes a smooth, sealed surface and a single coat. In our experience working with interior painters, real rooms have texture, doors, windows, and demand two coats for true color. Perimeter-based wall area, sensible opening deductions, and a shopping list you can trust, below.
Last updated: June 30, 2026
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.
10×10 vs larger rooms
A 10×10 ft bedroom has 40 ft perimeter and about 284 sq ft of paintable wall (8 ft ceilings, one door, one window), typically 2 gallons for two coats. That is less than a 12×12 room (about 348 sq ft paintable). Use our How Much Paint for a 10x10 Room guide for exact gallon math on the smaller footprint.
Frequently asked questions
- Does primer change the math?
- Use the same area formula with primer coverage from the can, often one coat.
- How much paint for a 10x10 room?
- About 2 gallons for two coats on walls with 8 ft ceilings, one door, and one window. Paintable wall area is roughly 284 sq ft, see our dedicated 10×10 room guide for the full step-by-step breakdown.