Floor area is not wall area. A reliable paint estimate begins with the room perimeter and wall height, subtracts substantial openings, and uses the exact product’s coverage rate before rounding up to cans.
Measure the paintable walls
Add the lengths of all walls for the perimeter and multiply by finished height. For a room with different ceiling heights, calculate each zone separately. Include alcoves only if they will receive the same coating.
Subtract large doors and windows when you have accurate dimensions. Small openings can be left in as a practical buffer, but state which approach you used.
Coverage is a product instruction
Coverage on a label is a starting point, not a promise for every wall. Texture, porosity, colour change, application method, and primer affect the result. Use the manufacturer’s rate for the exact product and surface, and round containers up rather than relying on a partial can.
Plan the coats
Multiply paintable area by the number of coats before dividing by coverage. A primer coat and finish coats may have different products and coverage; estimate them separately when that matters.
Older coatings and ventilation
Follow the product safety data sheet and provide ventilation. If sanding or disturbing paint in an older home, check local lead-safe guidance before starting; the EPA reference below explains why preparation can matter.
Worked examples
Bedroom with two windows
- Perimeter 42 ft × 8 ft height = 336 ft².
- Subtract a 21 ft² door and two 15 ft² windows: 285 ft².
- Two coats at 350 ft²/gallon require 570 ÷ 350 = 1.63 gallons.
Result: Round up to 2 gallons and verify the product’s label and colour-change recommendation.
Open-plan wall zone
- Perimeter zone 30 ft × 9 ft = 270 ft².
- Subtract a 60 ft² opening = 210 ft².
- At 400 ft²/gallon for one coat, 210 ÷ 400 rounds up to one gallon.
Result: If a second coat is needed, repeat the coverage calculation rather than assuming the first coat doubles itself.
Safety and responsibility
Use ventilation and the product safety information. Take extra precautions before disturbing older coatings.