How to use this calculator
- Measure room. Length and width in feet, plus ceiling height.
- Subtract openings. Count doors (≈ 21 ft² each) and windows (≈ 15 ft² each).
- Choose coats. Most jobs need 2 coats. Dark-over-light or stained surfaces may need 3.
- Read result. Round up to the nearest gallon — partial gallons stop being economical fast.
Formula
Gallons = ceil((wall area − doors − windows) × coats ÷ 350)
Worked example
A 12 × 14 ft room with 8 ft ceilings, 1 door and 2 windows needs 416 ft² of paintable area. Two coats = 832 ft² ÷ 350 = 3 gallons.
Tips for accurate results
- 1 US gallon covers ≈ 350 ft² in one coat on smooth drywall.
- Textured surfaces (popcorn, stucco) cover 250 ft²/gal — buy 30% more.
- Always buy paint in the same batch number to avoid color shifts.
- Save the empty cans — the label has the exact color formula for touch-ups.