Paint or Stain Coverage Calculator
Estimate how many gallons of paint or stain you need for walls, ceilings, fences, or decks — accounting for multiple coats and waste.
Formulas Used
Wall Area (rooms):
Wall Area = 2 × (Length + Width) × Height
Flat Surface Area (decks, ceilings, floors):
Flat Area = Length × Width
Net Paintable Area:
Net Area = Gross Area − (Doors × 21 sq ft) − (Windows × 15 sq ft)
Gallons Required (before waste):
Gallons = (Net Area × Number of Coats) ÷ Coverage Rate (sq ft/gal)
Gallons Required (with waste):
Gallonsfinal = Gallons × (1 + Waste Factor)
Purchase Quantity:
Round up to the nearest quart (0.25 gal) to avoid running short.
Assumptions & References
- Standard interior paint coverage is 350–400 sq ft per gallon on smooth surfaces (EPA / paint manufacturer data).
- Exterior paint and stains typically cover 200–300 sq ft per gallon due to surface porosity; adjust the coverage field accordingly.
- A standard door is assumed to be 21 sq ft (3 ft × 7 ft) and a standard window 15 sq ft (3 ft × 5 ft).
- Door and window deductions apply only in wall/room mode (when height > 0). Flat surfaces (decks, ceilings) have no deductions.
- Waste factors follow industry guidelines: 10–15% for smooth surfaces, up to 25% for rough or highly porous surfaces (e.g., bare wood, brick, stucco).
- Two coats are recommended for most projects to achieve full, even color coverage.
- Dark-to-light color changes may require a primer coat — add a separate calculation for primer if needed.
- Results are rounded up to the nearest quart to ensure you purchase enough product.
- Reference: Painting and Decorating Craftsman's Manual, Painting and Decorating Contractors of America (PDCA).