How this is calculated
Wall area starts from the room's perimeter (2 × (length + width)) multiplied by ceiling height, then subtracts a standard allowance for each door and window — painters call this the 'gross-to-net' area, and skipping the deduction is the most common reason people over-buy paint.
Paint coverage is quoted per coat, so the net wall area is multiplied by the number of coats before converting to gallons or litres using a standard coverage rate. Textured walls, unprimed drywall, and strong color changes all reduce real coverage below the can's rated figure.
The fixed opening deductions are planning values, not measurements of every door and window. For patio doors, bay windows, or a mostly glazed wall, calculate the opening area directly and adjust your room takeoff.
Paint is purchased in whole containers, so round up to an available tin size and keep enough from the same batch for continuous walls. Primer and ceiling paint should each be calculated as separate products with their own coverage rates.
Formula & methodology
Net area = (2 × (Length + Width) × Height) − (Doors × 21 ft² + Windows × 15 ft²). Paint needed = Net area × Coats ÷ 350 ft² per gallon.
Worked example
A 12 ft × 10 ft room, 8 ft ceilings, 1 door, 2 windows, 2 coats
- 1.Perimeter = 2 × (12 + 10) = 44 ft
- 2.Gross wall area = 44 ft × 8 ft = 352 ft²
- 3.Deductions = (1 × 21) + (2 × 15) = 51 ft²
- 4.Net area = 352 − 51 = 301 ft²
- 5.Paint needed = 301 × 2 coats ÷ 350 ft²/gal = 1.72 gal → round up to 2 gallons