How Much Paint Do You Need for a Room? A Step-by-Step Method

Step 1: find the wall area

For a rectangular room, the walls form a loop. Add up the lengths of all four walls (the perimeter) and multiply by the ceiling height.

wall area = 2 × (length + width) × heightUse the same unit for all three measurements.

Worked example: a room 12 ft by 14 ft with 8 ft ceilings. The perimeter is 2 × (12 + 14) = 52 ft, so the gross wall area is 52 × 8 = 416 ft².

Next, subtract the parts you will not paint. A standard interior door is roughly 3 ft × 6.7 ft, about 20 ft². A window 3 ft by 4 ft is 12 ft². With one door and two such windows, the openings total 20 + 24 = 44 ft², leaving 416 − 44 = 372 ft² of wall. Many painters skip subtracting very small openings, which builds in a little spare paint.

Step 2: coats and coverage

Most walls get two coats, especially when changing colour. Multiply the net area by the number of coats, then divide by the coverage on the label. Many cans quote roughly 350 to 400 ft² per US gallon; use the lower figure if you want a safety margin.

paint needed = net area × coats ÷ coverage per unit of paint

Continuing the example: 372 × 2 coats = 744 ft². At 350 ft² per gallon, that is 744 ÷ 350 = 2.13 gallons. At 400 ft² per gallon it is 1.86 gallons. Either way, two gallons is cutting it fine, so the practical purchase is 2 gallons plus a quart (2.25 gallons) or 3 gallons if you like a larger reserve.

In metric, the same formula gives litres. The room above is about 34.6 m² of wall. Two coats cover 69.1 m². If the label says 12 m² per litre, you need 69.1 ÷ 12 = 5.8 litres, so a 5 L tin plus a 1 L tin.

Paint is sold in fixed can sizes, so always round up to the nearest combination of cans instead of buying the exact figure. A small amount left over is useful insurance for future touch-ups and repairs, and it costs far less than running short on the last wall.

Ceilings, trim and primer

A ceiling is just the floor area: 12 × 14 = 168 ft². One coat at 350 ft² per gallon needs 0.48 gallon, and a second coat doubles that. Ceiling paint is often a different product, so calculate it separately rather than adding it to the wall total.

Trim, skirting boards and door frames use little paint, and are often covered by leftovers. If you are painting a lot of woodwork, treat each piece as a thin strip: length × width, then add those areas together.

Primer is calculated the same way, using the primer's own coverage figure. Bare plaster, new drywall and patched areas can soak up more paint than a sealed, previously painted wall.

Why the label is only an estimate

Coverage on a can assumes a smooth, sealed surface and an average application. Several things reduce it:

• Rough, textured or porous surfaces absorb more paint per square foot.
• A big colour change, such as dark to light, may need an extra coat.
• Rollers, sprayers and brushes lay down different amounts, and spraying loses some paint as overspray.
• Paint that is thinned beyond the maker's instructions covers less well and may need extra coats.

When in doubt, lean toward the lower coverage number and keep any leftover can sealed for touch-ups. Where a colour is mixed to order, buy everything for the room in one go so the cans come from the same batch, or combine them in a larger bucket before you start.

To check your wall area quickly, try the paint calculator, or work out the loop length with the perimeter of a rectangle calculator and the floor and ceiling with the square footage calculator.