Estimating interior paint quantities requires balancing total wall surface area against surface porosity, color transition contrast, and application method.
Running out of paint halfway through a wall forces an emergency trip to the paint store where subtle tint formula differences between batches can create noticeable color banding across your walls.
Here is the contractor formula to calculate interior paint and primer needs down to the exact can.
The Wall Area Formula & Standard Openings Deduction
- Calculate Gross Wall Area: Perimeter (Length + Width + Length + Width) × Ceiling Height. Example: A 12 ft × 15 ft bedroom with 8 ft ceilings has a perimeter of 54 ft. Gross wall area = 54 × 8 = **432 square feet.
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Deduct Standard Openings:
- Standard Interior Door: Deduct 21 sq ft.
- Standard Window: Deduct 15 sq ft.
- Large Sliding Glass Patio Door: Deduct 40 sq ft.
- Deducting 2 doors (42 sq ft) and 2 windows (30 sq ft) leaves 360 net square feet.
Coverage Rates: Standard vs. Porous Surfaces
- Smooth, Previously Painted Walls: 1 gallon covers 350 to 400 square feet.
- Textured Drywall / Unprimed Surfaces: 1 gallon covers 250 to 300 square feet.
- Ceiling Paint: 1 gallon covers 350 square feet (always calculate ceiling area separately: Length × Width).
When to Use a Dedicated Primer Coat
- New Bare Drywall: Always apply 1 dedicated coat of PVA drywall primer to seal porous mud and paper.
- Drastic Color Transitions (Dark to Light): 1 coat of high-hiding white primer prevents dark tones from bleeding through.
- Stains (Water or Smoke): Use an oil-based or shellac-based stain blocking primer.
👉 Calculate your exact wall gallons, ceiling cans, and primer requirements instantly with the free CraftCalc Paint Estimator.
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