This calculator determines the usable wall area for drywall installation, accounting for windows and door openings. **Gross Wall Area** = Perimeter (ft) × Height (ft) The gross area is the total wall surface if there were no windows or doors. This represents every wall surface in the room from floor to ceiling. **Total Opening Area** = Window Area (sq ft) + Door Area (sq ft) Measure all windows and doors and sum their areas in square feet. Each opening reduces the amount of drywall needed. **Net Wall Area** = Gross Area − Opening Area The net area is what you'll actually need to cover with drywall sheets, mud, tape, primer, and paint. This is the critical number for material ordering and cost estimation. The net area directly determines how many drywall sheets you need (typically 4'×8' or 4'×12' sheets at ~32 sq ft per sheet for a 4'×8'). **Example:** A 10×10 room with 8-foot ceilings: Gross = 40 × 8 = 320 sq ft. With 40 sq ft of openings: Net = 320 − 40 = 280 sq ft needed.