To calculate wall area, measure the perimeter of your room (2 × length + 2 × width) and multiply by the ceiling height. This gives you the total wall surface area. Then subtract the area of doors, windows, and other openings. A standard door is about 20 sq ft (1.86 sq m), and a standard window is about 12 sq ft (1.11 sq m). Our calculator does all of this automatically.

On average, one gallon (3.785 liters) of paint covers approximately 350 to 400 square feet (32 to 37 square meters) per coat on smooth, primed surfaces. Coverage varies based on surface texture, porosity, paint quality, and application method. Rougher surfaces like stucco or unpainted drywall may reduce coverage to 250-300 sq ft per gallon. Always check the manufacturer's label for specific coverage information.

Most painting projects require at least two coats for even, durable coverage. If you're painting over a similar color with high-quality paint, one coat may suffice. Painting over dark colors with lighter ones, or painting new drywall, typically requires 2-3 coats. Using a primer can reduce the number of finish coats needed and improve adhesion.

Yes! We recommend purchasing 10-15% more paint than the exact calculation suggests. This accounts for touch-ups, spills, absorption variations, and ensures you have matching paint from the same batch. Having extra paint is especially important for future touch-ups, as color matching between different batches can vary slightly. Our calculator includes a recommended purchase amount with a 10% buffer.

Only if you plan to paint the ceiling. Ceiling area is simply length × width of the room. If you're painting the ceiling the same color as the walls, check the "Include ceiling area" option. If using a different color or finish, calculate the ceiling separately. Ceilings often require different paint (flat/matte finish) than walls, so factor that into your planning.

Key factors include: surface texture (rough surfaces absorb more paint), porosity of the substrate (new drywall is very absorbent), paint quality (higher-quality paints have better hide and coverage), application method (rollers vs. sprayers), color change (dramatic color changes may need extra coats), and whether a primer was used. Temperature and humidity during application can also affect coverage and drying.

1 US gallon equals approximately 3.785 liters. 1 square foot equals 0.0929 square meters. A typical coverage of 350 sq ft per gallon translates to roughly 8.5-9 square meters per liter. Our calculator handles both imperial (feet, gallons, square feet) and metric (meters, liters, square meters) units. Simply toggle between the two systems using the switch at the top.

A standard interior door is approximately 80 inches tall by 36 inches wide (203 × 91 cm), which equals about 20 square feet (1.86 sq m). Standard windows vary, but a common size is 48 × 36 inches (122 × 91 cm), about 12 square feet (1.11 sq m). For larger sliding doors or bay windows, measure the actual dimensions. Our calculator uses these defaults but allows you to customize the area per opening.