How to Calculate Wall Area for Painting & Wallpapering

Step by Step

1. Measure room dimensions. Record length, width, and ceiling height in feet.
2. Calculate perimeter. Add the lengths of all walls: 2 × (length + width).
3. Multiply by height. Perimeter × ceiling height = gross wall area.
4. Subtract doors and windows. Standard door: 21 sq ft. Standard window: 15 sq ft. Custom sizes: measure and subtract actual area.
5. Add ceiling separately if needed. Ceiling area = length × width. Paint or wallpaper ceilings as their own job.

Worked Example

Bedroom: 12 × 14 ft, 8 ft ceiling, 1 door, 2 windows.

• Perimeter = 2 × (12 + 14) = 52 ft
• Gross wall area = 52 × 8 = 416 sq ft
• Openings = 21 + 2 × 15 = 51 sq ft
• Net wall area = 416 − 51 = 365 sq ft

For two coats: 365 × 2 = 730 sq ft of paintable area. At 350 sq ft per gallon, you need 2.1 gallons — order 2.5 gallons (a gallon plus a quart) to cover touch-ups.

Standard Opening Sizes

• Interior door: 3 × 7 ft = 21 sq ft
• Exterior door: 3 × 7 ft = 21 sq ft (sometimes 36 × 80 in)
• Double front door: 6 × 7 ft = 42 sq ft
• Standard window: 3 × 5 ft = 15 sq ft
• Large picture window: 6 × 4 ft = 24 sq ft
• Sliding glass door: 6 × 7 ft = 42 sq ft
• Closet pocket door: 2.5 × 7 ft = 17.5 sq ft

Adjustments for Tricky Walls

Textured Surfaces

Texture increases actual surface area: light orange peel +15%, knockdown +25%, popcorn +30%. Multiply your smooth-wall area by these factors to estimate paint and primer.

Vaulted or Slanted Ceilings

Triangular gable walls: area = (base × height) ÷ 2. Trapezoidal walls (slanted ceiling): area = ((top + bottom) / 2) × wall height. Add these to your perimeter × straight-wall height.

Stairwells

The triangular wall along a stairwell has area = (run × rise) ÷ 2 for one side. For both sides combined: run × rise.