Paint Calculator

How the paint calculator works

Paint is sold by coverage: the label tells you how much surface a gallon or litre covers in one coat. So the job is to find the paintable area, multiply by the number of coats, and divide by that coverage figure. The calculator does this for the four walls of a rectangular room, subtracts doors and windows, and optionally adds the ceiling and a coat of primer.

The default coverage is deliberately cautious. Sherwin-Williams says a gallon typically covers about 350–400 ft², and that a gallon of primer covers about 200–300 ft². In the UK, Dulux lists its Matt emulsion at “up to 14 m² per litre”. “Up to” describes smooth, sealed walls in good conditions, so the metric default here is 10 m²/L. Type in the figure from your own tin for the best answer.

The formula

Wall area   = 2 × (length + width) × height − doors − windows
Paint       = wall area × coats ÷ coverage per coat
Ceiling     = length × width × coats ÷ coverage
1 US gallon = 3.785 litres      1 quart = ¼ gallon

In imperial mode the result is rounded up to whole gallons plus quarts. In metric mode it suggests the combination of 5 L, 2.5 L and 1 L tins that covers the need with the fewest leftover litres. A standard 36 × 80 in door is 20 ft², and 15 ft² is a common allowance for an average window. Change either if your openings are bigger or smaller.

Worked examples

A 12 × 12 ft bedroom with 8 ft walls

Walls: 2 × (12 + 12) × 8 = 384 ft². Subtract one door (20 ft²) and two windows (30 ft²) to get 334 ft². Two coats at 350 ft²/gal need 668 ÷ 350 = 1.91 gallons, so buy 2 gallons. The ceiling is 144 ft²; two coats need 0.82 gallons, so one more gallon of ceiling paint.

A 4 × 3.5 m living room with 2.4 m walls (metric)

Walls: 2 × (4 + 3.5) × 2.4 = 36 m², less a 1.9 m² door and a 1.4 m² window = 32.7 m². Two coats at 10 m²/L need 6.5 L, so a 5 L tin plus two 1 L tins (7 L). At the label’s best case of 14 m²/L you would need about 4.7 L. That’s why Dulux’s in-store rule of thumb of about 5 litres for a 4 × 4 m room can work on smooth walls, but leaves little margin on rough or porous ones.

What changes how much paint you need

• Surface. Textured, porous or freshly plastered walls soak up more paint. New plaster usually needs a mist coat or primer first; follow the paint maker’s instructions.
• Colour change. Dulux notes that a strong colour change may need more coats. Going from dark to light is the classic case where a primer or an extra coat pays off.
• Application. Sprayers use more paint than rollers because of overspray; the rates above assume brush and roller.
• Trim and doors are not included. Sherwin-Williams says a quart covers about 100 ft², which is usually plenty for one room’s doors and skirting/baseboards.

Practical tips

• If you need more than one can of the same colour, Dulux suggests mixing the cans together in a large container, or finishing a wall before opening the next can, so small colour differences don’t show.
• Keep a little leftover, labelled with the room and date, for touch-ups.
• Measure the actual wall height. Many homes have 8 ft (2.4 m) ceilings, but older and newer houses often differ.