Paint Calculator

Paint manufacturers state a coverage rate: the area one gallon or one litre covers in a single coat. This calculator measures the paintable wall area, multiplies by the number of coats and divides by that coverage rate.

Coverage is editable because it genuinely varies. Labels often quote somewhere around 250–400 sq ft per US gallon (roughly 6–10 m² per litre) for wall paints, and rough, porous or freshly plastered surfaces absorb more. Use the figure from the product you plan to buy.

1. Choose how to measure: enter one wall width and the number of walls (good for square rooms), or the room perimeter (the sum of all wall lengths).
2. Enter the wall height from floor to ceiling.
3. Enter the number and size of doors and windows. Set a count to 0 to skip it.
4. Set the number of coats and the coverage from the paint can or data sheet.
5. Choose the container size you will buy and, optionally, its price.

Wall area = Wall width × Wall height × Number of walls

Paintable area = Wall area − (Doors + Windows)

Paint needed = Paintable area × Coats ÷ Coverage

Containers = Paint needed ÷ Container size, rounded up

If coverage is given in square metres per litre and you measured in feet, the calculator converts automatically: 1 sq ft = 0.0929 m² and 1 US gallon = 3.785 litres. Note that a UK (imperial) gallon is 4.546 litres, but UK paint is sold in litres.

A 12 ft × 12 ft bedroom with 8 ft walls, one door and two windows, painted with two coats:

• Wall area: 4 walls × 12 ft × 8 ft = 384 sq ft
• Door (3 × 7 ft) = 21 sq ft; windows (2 × 3 × 4 ft) = 24 sq ft
• Paintable area: 384 − 45 = 339 sq ft
• Two coats: 678 sq ft
• At 350 sq ft per gallon: 678 ÷ 350 = 1.94 gallons → buy 2 one-gallon cans

Metric: a room with a 14.4 m perimeter and 2.4 m walls has 34.56 m² of wall. Subtracting a 0.82 × 2.04 m door and two 1.2 × 1.2 m windows leaves 30.0 m². Two coats at 10 m²/L needs 6.0 litres, or three 2.5-litre tins.

• Paint needed – total volume for all coats, in US gallons or litres.
• Containers – the paint needed divided by your container size, always rounded up.
• Paintable area – wall area after subtracting doors and windows; the area of one coat.
• Area to cover – paintable area multiplied by the number of coats.

• Buy all the paint for a room from the same batch, or mix (“box”) cans together to keep the colour even.
• Small openings such as switch plates are not worth subtracting.
• Keep leftover paint, labelled with the room and date, for touch-ups.
• Calculate ceilings separately: their area is the room length × width.
• Mixing container sizes can save money, for example one gallon plus one quart instead of two gallons.

• Surface – new drywall, bare plaster and masonry soak up paint; primer or a mist coat is often recommended.
• Texture – textured walls have more surface area than flat ones.
• Colour change – going from dark to light usually needs an extra coat.
• Application – rollers with a thick nap and spraying both use more paint than the label figure.

Hanging new drywall first? The drywall calculator uses the same wall measurements. Our guide How much paint do you need for a room? walks through a full room.