Paint Calculator
Updated 2026-10-12
Enter your room's size and this calculator subtracts doors and windows, multiplies by the number of coats, and tells you how much paint to buy.
How the paint calculator works
A room's walls form a rectangle when you "unroll" them, so the total wall area is the distance around the room (the perimeter) multiplied by the wall height. From that we remove the parts you won't paint, then multiply by the number of coats and divide by how far one gallon (or liter) spreads.
Paint needed = area × coats ÷ coverage
The calculator assumes a standard door of about 21 sq ft (3 × 7 ft, or 1.9 m²) and a standard window of about 15 sq ft (3 × 5 ft, or 1.4 m²). If you have a large patio door or a picture window, add an extra door or window to account for it.
Worked example
A 12 × 10 ft bedroom with 8 ft walls has a perimeter of 44 ft, so the walls cover 352 sq ft. Take away one door (21 sq ft) and two windows (30 sq ft) and you are left with 301 sq ft. Two coats double that to 602 sq ft. At 350 sq ft per gallon, you need about 1.72 gallons, so one gallon plus three quarts, or simply two gallons.
How far does a gallon of paint go?
Coverage depends on the paint and the surface. The number printed on the can assumes a smooth, already-painted surface. Rough or thirsty surfaces soak up more.
| Surface | Typical coverage per gallon | Per liter |
|---|---|---|
| Smooth, previously painted drywall | 350–400 sq ft | 9–11 m² |
| New drywall (with primer) | 250–300 sq ft for primer | 6–7 m² |
| Textured walls or ceilings | 200–300 sq ft | 5–7 m² |
| Bare wood or masonry | 150–250 sq ft | 4–6 m² |
If your surface is textured or unpainted, lower the coverage figure in the calculator to match.
One coat or two?
Two coats are the norm. The first coat almost always shows brush marks and patchy spots, and the second evens out the color and sheen. You may get away with one coat when you repaint a wall in the same or a very similar color with a good-quality paint. A dramatic color change, especially light over dark, often needs a primer plus two coats.
Tips to avoid buying too much or too little
- Buy all the paint at once. Tints can vary slightly between batches, so running short halfway through a wall can leave a visible line.
- Keep some for touch-ups. A spare quart or liter, labeled with the room and color, is useful for scuffs later.
- Measure, don't guess. A tape measure and two minutes give you a far better estimate than eyeballing the room.
- Ceilings usually need separate paint. Most people use a flat ceiling white, so only select "paint the ceiling too" if it will be the same paint as the walls.
Paint for doors and trim
Doors, door frames, window frames and baseboards are usually painted in a harder-wearing semi-gloss or satin finish. They take far less paint than the walls. A single quart (about 1 liter) covers the trim of an average bedroom, and one side of a standard interior door uses roughly a third of a quart for two coats. If you are repainting all the doors in a house, a gallon is usually plenty.
Frequently asked questions
- How much paint do I need for a 12 × 12 room?
- With 8 ft walls, one door and two windows, the walls come to about 333 sq ft. Two coats at 350 sq ft per gallon need roughly 1.9 gallons, so buy two gallons.
- Should I include the trim and baseboards?
- No. Trim is usually painted in a different sheen, such as semi-gloss, and uses very little paint. A quart covers the trim of most rooms.
- Does primer count as a coat?
- Primer is a separate product. Work out primer with the calculator set to one coat and the primer's own coverage figure, then work out the finish paint separately.