How Much Paint Do I Need for a Room?

Step 1: measure the wall area

Add up the lengths of all the walls (the perimeter) and multiply by the wall height. A 12 × 14 ft room with 8 ft ceilings has a perimeter of 52 ft and a wall area of 416 sq ft. In metric, a 4 × 3.5 m room with 2.4 m walls has a perimeter of 15 m and a wall area of 36 m².

Then deduct openings. A standard interior door is about 21 sq ft (1.9 m²) and an average window about 15 sq ft (1.4 m²). For the 12 × 14 ft room with one door and two windows: 416 − 21 − 30 = 365 sq ft.

Step 2: choose coverage and coats

Coverage is the area one gallon or liter covers in one coat. Interior wall paints typically state 350–400 sq ft per US gallon, or 10–12 m² per liter for good-quality emulsion. Rough, porous or new surfaces absorb more, so the real figure is often lower. We default to 350 sq ft per gallon and 8.5 m² per liter to stay on the safe side.

Two coats is the norm for an even finish, especially over a different color. A single coat may be enough when repainting the same color. Bare plaster and drywall need a mist coat or primer first.

Step 3: calculate volume

Volume = area × coats ÷ coverage. For 365 sq ft and two coats at 350 sq ft per gallon: 365 × 2 ÷ 350 = 2.09 gallons. For the metric room, 36 m² less openings (say 32 m²), two coats at 8.5 m²/L: 32 × 2 ÷ 8.5 = 7.5 L.

Step 4: buy the right tins

Paint is sold in fixed sizes, so round up to a sensible combination. 2.09 gallons becomes two gallons plus one quart — or three gallons if a third gallon costs little more than a quart. 7.5 L could be a 5 L plus a 2.5 L tin. Our paint calculator suggests the combination with the least leftover, and you can adjust for price.

Ceilings are usually painted separately in flat white. A ceiling is roughly the floor area: 168 sq ft for the 12 × 14 room, needing about one gallon for two coats.

Trim, doors and accent walls

• Skirting, door frames and window trim are usually painted in a different sheen; a quart or 750 ml tin covers a typical room's trim.
• Both sides of a door: about 40 sq ft (3.8 m²) per coat.
• For an accent wall, measure that wall alone and note that strong colors often need a third coat or a tinted primer.

Surfaces that use more paint

Coverage figures assume smooth, sealed walls. Textured finishes such as knock-down, popcorn or Artex increase the true surface area and can reduce coverage by 10–30%. Fresh plaster and bare drywall soak up the first coat, which is why a diluted mist coat or a primer goes on first. Masonry and exterior render are even thirstier; use the coverage on the masonry paint tin, which is often much lower than for interior emulsion.

Color also matters. Deep reds, yellows and very dark colors often need three coats over a light base, or a gray-tinted primer. Moving from a dark wall to white usually needs a stain-blocking or high-hiding primer first. When you change color dramatically, add a coat in the calculator rather than hoping for better coverage.

Worked example: a UK bedroom

1. Room 3.6 × 3.0 m, walls 2.4 m high: perimeter 13.2 m, wall area 31.7 m².
2. One door (1.9 m²) and one window (1.4 m²): 31.7 − 3.3 = 28.4 m².
3. Two coats at 10 m²/L (from the tin): 28.4 × 2 ÷ 10 = 5.7 L → a 5 L tin plus a 1 L tin, or 2 × 2.5 L tins plus a 1 L tin.
4. Ceiling 10.8 m², two coats at 10 m²/L: 2.2 L → a 2.5 L tin of white matt.

Leftovers

Seal leftover paint tightly and store it upside down to prevent a skin forming. Many areas run paint reuse or recycling schemes; do not pour paint into drains.

Tips for an accurate estimate

• Read the coverage on the actual tin you plan to buy and enter it in the calculator.
• Box several tins together for an even color.
• Keep a small labelled jar of leftover paint for touch-ups.
• Do not deduct small openings like switch plates — they make no measurable difference.