Measure and units
Use one unit system from start to finish. Total wall area equals width multiplied by height. For multiple walls, calculate each rectangle and add the results before subtracting openings.
Openings and net area
Subtract doors, windows, and large built-ins that will not be painted or covered. If there are multiple openings, sum their individual areas. The calculator prevents negative net area when openings are larger than the wall.
Paint coverage and coats
Typical paint coverage is about 350 ft² per gallon or 8.6 m² per liter on smooth primed drywall. Rough surfaces, deep colors, low-sheen finishes, and poor primer can reduce coverage. Two coats double the paint amount.
Wallpaper rolls and patterns
Use 56 ft² or 5.2 m² per standard roll as a quick planning rule. Pattern repeat can increase waste because sheets must align. For straight match, allow around 10% extra; for drop match, allow up to 15% or more.
Prep and safety
- Repair dents, sand smooth, and wipe dust before coating.
- Mask trims and protect floors before paint or paste work.
- Ventilate the room and follow dry time guidance from the product label.
- Use the primer recommended for stains, new drywall, glossy enamel, or drastic color changes.
Worked examples
A 15 by 10 ft wall equals 150 ft². One 3 by 7 ft window equals 21 ft², so net area is 129 ft². Two coats need about 0.74 gallons at 350 ft² per gallon. Wallpaper needs 3 rolls after rounding, plus a spare roll when pattern matching matters.
A 4.5 by 2.7 m wall equals 12.15 m². One 0.9 by 1.8 m window equals 1.62 m², so net area is 10.53 m². One coat needs about 1.22 liters at 8.6 m² per liter; two coats need about 2.44 liters.
References
Wikipedia Paint | Wikipedia Wallpaper | Wikipedia Primer | Square foot | Square metre