How to use this calculator
- Enter the room's length, width and wall height.
- Enter the number of doors and windows to subtract.
- Choose the number of coats (two is standard, especially for color changes) and the coverage from the paint can.
Formula
Wall area is the room's perimeter times wall height, minus openings. Standard openings are about 21 ft² (1.9 m²) per door and 15 ft² (1.4 m²) per window.
Wall area = 2 × (L + W) × H − doors − windows
Paint = area × coats ÷ coverage
Example
- 14 × 12 ft room, 8 ft walls: perimeter 52 ft → 416 ft².
- Minus 1 door and 2 windows ≈ 365 ft².
- 2 coats at 350 ft²/gal = 2.09 gal, so buy 2 gallons plus a quart, or 3 gallons.
Frequently asked questions
How much does a gallon of paint cover?
Usually 350–400 ft² (about 9–10 m² per liter) for one coat on smooth, primed walls. Rough or porous surfaces need more.
Do I need primer?
Use primer on new drywall, bare wood, stains, or when making a dramatic color change. Primer coverage is similar to paint.
Should I buy extra?
Keep some left over for touch-ups, ideally from the same batch, since tints can vary.