Paint calculator for a room
Enter room size, doors, windows, coats and coverage to get the wall area, gallons (or liters) of paint needed and the cans to buy.
- Wall area to paint
- —
- Ceiling area
- —
- Suggested cans
- —
Assumptions: one door = 20 sq ft, one window = 15 sq ft. Real coverage depends on the surface (new drywall, covering a dark colour…): follow the product label.
- Gallons = paintable area (sq ft) × coats ÷ coverage per gallon.
- Sherwin-Williams says one gallon typically covers 350–400 sq ft per coat on a smooth, prepared surface.
- Most rooms need two coats; add about 10% for touch-ups.
The calculation in three steps
- Wall area = room perimeter × ceiling height. A 12 × 12 ft room with 8 ft ceilings: (12 + 12) × 2 × 8 = 384 sq ft.
- Subtract openings: about 20 sq ft per standard door and 15 sq ft per average window. With one of each: 384 − 20 − 15 = 349 sq ft.
- Multiply by coats and divide by coverage: 349 × 2 ÷ 350 ≈ 2.0 gallons, or about 2.2 gallons with a 10% allowance. In practice: two 1-gallon cans plus one quart.
What coverage figure should I use?
The coverage rate is printed on the can and on the product data sheet. Sherwin-Williams indicates that one gallon typically covers 350–400 sq ft per coat on a prepared, smooth surface. European wall paints usually state coverage in square meters per liter, often around 10–12 m²/L.
Real coverage drops on:
- porous surfaces such as new drywall or bare plaster (prime first);
- textured surfaces such as popcorn ceilings, stucco or rough masonry;
- big colour changes, especially light over dark, which may need a third coat.
If in doubt, enter a lower coverage (for example 300 sq ft per gallon). A little paint left over is better than buying a second batch that doesn’t quite match.
One coat or two?
Two coats are standard for even colour and durability. One coat can be enough to refresh a wall in the same colour with a good-quality paint. “Paint and primer in one” products still assume a sound, compatible surface.
Buying tips
- Buy all the paint from the same batch, or mix (box) the cans together before you start, to avoid slight colour differences.
- Five-gallon buckets are cheaper per gallon but only make sense for large areas or several rooms in the same colour.
- Keep a small amount, tightly sealed, for touch-ups.
Need the full method with more examples? Read how much paint do I need for a room.