How much paint do I need? Measuring a room properly

Buy too little paint and you end up making a second trip, hoping the new tin matches. Buy far too much and it sits in a shed for a decade. Working out the right amount takes a tape measure and five minutes.

What to measure

• The length and width of the room, along the floor.
• The height of the walls, from floor to ceiling.
• The doors and windows — how many, and roughly how big.

Measure in one unit throughout, metres or feet. If your tape gives you a mixture, the Length Converter converts it.

Work out the wall area

wall area = 2 × (length + width) × height

That is the distance around the room multiplied by the height. For a 4 × 3 metre bedroom with 2.5 metre walls: 2 × (4 + 3) × 2.5 = 35 m².

Then subtract the doors and windows. A standard internal door is roughly 1.9 m² and an average window around 1.5 m², though it is worth measuring large windows. One door and one window leave 35 − 1.9 − 1.5 = 31.6 m² to paint.

Coats and coverage

Every tin states its coverage, usually as square metres per litre or square feet per gallon. Interior wall paints typically cover about 10–14 m² per litre (roughly 350–400 square feet per US gallon) on a smooth surface that has been painted before.

Plan on two coats for almost every job. One coat is only enough when repainting in the same colour over a sound surface; a big colour change, especially dark to light, may need three.

paint needed = area × coats ÷ coverage, plus about 10% for waste and touch-ups.

A worked example (metric)

1. Wall area after the door and window: 31.6 m².
2. Two coats: 31.6 × 2 = 63.2 m².
3. At 12 m² per litre: 63.2 ÷ 12 = 5.27 litres.
4. Plus 10%: about 5.8 litres.

In practice that is one 5-litre tin and one 1-litre tin. The Paint and Wallpaper Calculator does these steps for you and suggests the combination of standard tin sizes that covers what you need with the fewest containers.

A worked example (imperial)

A 12 × 10 foot room with 8-foot walls: 2 × (12 + 10) × 8 = 352 square feet. Subtracting a typical 3 × 7 foot door (21 sq ft) and a 15 sq ft window leaves 316 square feet. Two coats at 375 square feet per gallon need 316 × 2 ÷ 375 = 1.69 gallons, or about 1.85 with 10% extra — so buy 2 gallons.

Things that need more paint

• New plaster soaks up paint. Use a thinned mist coat or a primer first, as the paint maker recommends.
• Rough or textured walls have more surface than their measurements suggest.
• Strong colour changes may need an extra coat or a tinted undercoat.

Ceilings and woodwork

A ceiling usually takes a different paint, so work it out separately: its area is simply length × width. The 4 × 3 m bedroom has a 12 m² ceiling, which at two coats and 12 m² per litre needs about 2.2 litres with the waste allowance. Skirting boards, door frames and window frames are usually painted in a different finish and need only a small tin.

Wallpaper is counted differently

Wallpaper is bought in rolls and hung in full-height strips called drops. How many drops a roll gives depends on the wall height and the pattern repeat — the distance before the pattern starts again, which has to line up from one strip to the next. A large repeat wastes more paper. Doors and windows are not usually subtracted, because offcuts rarely match the pattern. Switch the calculator to wallpaper to count rolls this way.

Before you buy

• Buy all the paint or paper for a room at the same time; batches can differ slightly in colour.
• Check the coverage figure on the actual product you are buying.
• Keep a little left over, labelled with the room and colour, for touch-ups.

Floor areas for carpets and flooring are calculated as length × width; the Area Converter switches between square metres and square feet.