Paint Calculator

Calculate how much paint you need for an interior room. Enter room dimensions, doors, windows, coats, and paint coverage to get an accurate gallon estimate with waste allowance.

ft

Interior length of the room.

ft

Interior width of the room.

ft

Ceiling height (standard: 8 ft).

Standard interior doors (~3×7 ft = 21 sq ft each). Defaults to 1.

Standard windows (~3×5 ft = 15 sq ft each). Defaults to 2.

Most walls need 2 coats for even coverage. Defaults to 2.

sq ft/gal

Square feet per gallon. Manufacturer average: 350. Check your paint can.

%

Extra paint to order for spillage, touch-ups, and roller waste. Defaults to 10%.

How this calculator works

This paint calculator works in five steps:

  1. Perimeter: 2 × (room length + room width) = total wall length.
  2. Gross wall area: perimeter × wall height.
  3. Openings deducted: standard interior doors (3×7 ft = 21 sq ft each) and windows (3×5 ft = 15 sq ft each) are subtracted from the gross wall area.
  4. Total paintable area: net area × number of coats (default 2).
  5. Gallons: total paintable area ÷ paint coverage (default 350 sq ft/gallon), then add waste allowance (default 10%).

The default coverage of 350 sq ft per gallon is the manufacturer average across Sherwin-Williams, Behr, and Benjamin Moore interior wall paints over a primed surface.

Formula

perimeter        = 2 × (room_length + room_width)
gross_wall_area  = perimeter × wall_height
door_area        = doors × 21 sq ft  (3×7 ft standard door)
window_area      = windows × 15 sq ft (3×5 ft standard window)
paintable_area   = gross_wall_area − door_area − window_area
total_paintable  = paintable_area × coats
gallons          = total_paintable / coverage
recommended      = gallons × (1 + waste_pct / 100)
  rounded up to nearest quart (0.25 gal)

Example

For a 12 × 10 ft room with 8 ft ceilings, 1 door, 2 windows, 2 coats, 350 sq ft/gal coverage, 10% waste:

  • perimeter = 2 × (12 + 10) = 44 ft
  • gross_wall_area = 44 × 8 = 352 sq ft
  • door_area = 1 × 21 = 21 sq ft
  • window_area = 2 × 15 = 30 sq ft
  • paintable_area = 352 − 21 − 30 = 301 sq ft
  • total_paintable = 301 × 2 = 602 sq ft
  • gallons = 602 / 350 = 1.72 gallons
  • recommended = 1.72 × 1.10 = 1.89 → 2 gallons

Important assumptions

  • Standard interior door: 3 × 7 ft = 21 sq ft (actual door size, not rough opening).
  • Standard window: 3 × 5 ft = 15 sq ft.
  • Default paint coverage: 350 sq ft/gallon — manufacturer average for interior wall paint over primed drywall.
  • Waste allowance default 10% covers roller absorption, spills, touch-ups.
  • This calculator estimates WALL paint only. For ceiling paint, multiply room length × width and use the coverage rate.
  • For textured walls (popcorn, knock-down), increase waste to 15% — textured surfaces absorb more paint.
  • Primer is calculated separately (200-300 sq ft/gal typical).

FAQ

How much paint do I need for a 12×12 room?
A 12×12 room with 8 ft ceilings, 1 door, 2 windows, and 2 coats needs about 1.8 gallons of paint — order 2 gallons to allow for waste and touch-ups.
How many square feet does a gallon of paint cover?
A gallon of interior wall paint typically covers 350 sq ft per coat over a primed surface. Coverage varies by paint quality, surface texture, and color — check the paint can label for the manufacturer's specific coverage rate.
Do I need 1 or 2 coats of paint?
Most walls need 2 coats for even coverage, especially when painting over a different color or using a light color over a dark one. If you're repainting with the same color in good condition, 1 coat may suffice.
Should I paint the ceiling too?
Ceiling paint is calculated separately. Multiply room length × width to get ceiling area, then divide by your coverage rate. Most ceilings use flat-finish paint, which covers slightly more than wall paint.
How do I account for a textured wall?
Textured walls (popcorn, knock-down, orange peel) absorb more paint. Increase the waste allowance to 15% and consider a second coat. You may also need to prime first if the texture is new.
Does this calculator include primer?
No. Primer is calculated separately — it typically covers 200-300 sq ft/gallon. If you're painting over bare drywall or a dramatically different color, plan for 1 coat of primer before your 2 coats of paint.