How to Calculate Interior Wall Paint
Painting estimation begins by finding the perimeter of the room and multiplying by the wall height, then subtracting unpainted openings like doors and windows:
Gross Wall Area = 2 × (Length + Width) × Wall Height
Openings Area = (Doors × 21 sq ft) + (Windows × 15 sq ft)
Net Paintable Area = Gross Wall Area - Openings Area
Gallons Needed = Round UP ( (Net Paintable Area × Number of Coats) ÷ 350 )
Openings Area = (Doors × 21 sq ft) + (Windows × 15 sq ft)
Net Paintable Area = Gross Wall Area - Openings Area
Gallons Needed = Round UP ( (Net Paintable Area × Number of Coats) ÷ 350 )
Standard Cutout Deductions
- Standard Interior Door: 21 sq ft (3 ft wide by 7 ft tall).
- Standard Window: 15 sq ft (3 ft wide by 5 ft tall).
- Sliding Patio Door: 40 to 48 sq ft (6 ft wide by 6.8 to 8 ft tall).
- Fireplace Mantel: 20 to 30 sq ft.
Why Two Coats Are Always Recommended
Even premium paints claiming "one-coat coverage" require two coats to guarantee manufacturer durability and uniform light reflectivity. Bare drywall absorbs paint unevenly, so a dedicated PVA primer coat is always recommended before applying the finish color.
Frequently Asked Questions
How do I calculate ceiling paint separately?
Ceiling area equals
Length × Width of the floor. Divide the ceiling area by 350 sq ft/gallon to find how many gallons of flat ceiling paint you need.What paint coverage rate should I use for textured walls?
For orange peel, knockdown, or masonry surfaces, reduce your coverage estimate from 350 to 275 - 300 square feet per gallon due to increased surface texture.