How Much Paint Do I Need? Coverage, Coats and the Sheen Chart

Paint is the cheapest change you can make to a room and the one people most often buy wrong — two litres short on a Sunday afternoon, or four litres too many in a colour that will never be used again.

The arithmetic takes two minutes.

The formula

Litres needed = (wall area − openings) × coats ÷ coverage per litre × 1.1

Where:

• Wall area = room perimeter × ceiling height
• Openings = about 1.8 m² (20 sq ft) per door, 1.5 m² (16 sq ft) per average window
• Coats = 2, almost always
• Coverage = 10–12 m² per litre, or 350–400 sq ft per US gallon
• × 1.1 = 10% for waste, spills and touch-ups

Ceilings are easier: ceiling area = floor area.

Worked example

A 4 × 4 m room with a 2.4 m ceiling, one door and one window:

1. Perimeter = 16 m. Wall area = 16 × 2.4 = 38.4 m²
2. Minus a door (1.8) and a window (1.5) = 35.1 m²
3. Two coats = 70.2 m²
4. At 11 m²/litre = 6.4 litres, plus 10% = 7 litres → buy 7.5 L (a 5 L plus a 2.5 L)
5. Ceiling = 16 m² × 2 coats ÷ 11 = 2.9 litres → buy 3 L

In US units, the same room is roughly 13 × 13 ft with an 8 ft ceiling: about 378 sq ft of wall after openings, 756 sq ft over two coats, so 2 gallons for the walls and 1 gallon for the ceiling.

Quick reference: paint per room

Assumes 2.4 m / 8 ft ceilings, two coats, one door and one window already deducted.

For trim and joinery, a rough guide: 1 litre covers about 60 metres of standard skirting per coat, and a standard door takes roughly 0.35 litres per coat for both faces and edges.

When two coats is not enough

Plan a third coat, or a tinted primer, in these cases:

Tinted primer is the underrated move here. A primer mixed towards your finish colour costs less than a litre of premium paint and routinely turns a three-coat job into two.

The sheen chart

Sheen is a durability decision that happens to be visible. The glossier the paint, the tougher and more wipeable it is — and the more mercilessly it shows every ripple in the wall.

Three practical rules:

1. Ceilings in flat matt, always. Any sheen on a ceiling turns every light source into a reflection that maps the plaster.
2. Trim one or two steps glossier than the walls. The contrast is what makes trim read as trim.
3. A wall in bad condition needs a flatter sheen, not more coats. Sheen amplifies texture; matt absorbs it.

Colour, and the mistake of testing on the wall

Coverage is the easy half. The hard half is that a colour behaves differently in your room than in the shop, and the difference is dominated by light.

• Test in the room, at night as well as by day. Most rooms are used after dark under 2,700K bulbs, and that is not what the shop showed you.
• Paint the sample on a board, not the wall. A patch of new colour sitting in a field of the old one is read by your eye against the old colour, which shifts your perception of it. A movable A4 board lets you check it against the sofa, the floor and the window wall.
• Two coats on the sample. A single-coat test is a different colour.
• Check it against the fixed things. Floor tone and worktop undertone constrain a palette far more than furniture does.

The interaction between paint and lighting is stronger than most people expect — the same warm white can read cream under 2,700K and clinical grey under 4,000K. The lighting by the numbers guide covers which temperature to standardise on, and most popular paint colours covers the shades themselves and their undertones.

Prep, in the order that matters

Most bad paint jobs are bad preparation, not bad paint.

1. Wash the walls. Sugar soap or a mild detergent, then rinse. Paint does not stick to kitchen grease or bathroom film.
2. Fill and sand. Fill, let it dry fully, sand flush, dust off with a damp cloth.
3. Spot-prime repairs. Otherwise the patches “flash” — visible dull spots under the finish coat.
4. Caulk the gaps where trim meets wall. This one step is the difference between “painted” and “decorated”.
5. Cut in one wall at a time, then roll it while the cut-in edge is still wet, or you get a visible frame of different texture.
6. Roller nap matters: 4–6 mm for smooth walls, 10–12 mm for textured surfaces. Too long a nap on a smooth wall adds an orange-peel texture you cannot undo.

Conditions: 10–25 °C, humidity below about 70%, and ventilation rather than a heater blasting one side of the room.

Estimating cost

Premium paint frequently costs less per finished wall than budget paint, because it covers in two coats where the cheap tin needs three. That advantage is largest exactly where opacity is hardest: deep and saturated colours.

For a whole-home plan rather than one room, the whole-house colour scheme guide sets out how to keep adjacent rooms in one family.

See the colour on your own walls first

The reason people buy the wrong paint is not arithmetic. It is that a 10 × 10 cm chip cannot tell you what an entire room will feel like — and by the time you know, the walls are already that colour.

Architectural AI repaints a photo of your actual room in any shade, so you can compare a warm white, a deep green and a clay tone on your own walls, with your own light and your own furniture in frame, before you buy a single tin. There is a step-by-step in test paint colours on a photo of your wall, and accent wall ideas covers which wall to pick if you are only painting one.

Frequently asked questions

How much paint do I need for a 12x12 room?

A 12 × 12 ft room (3.7 × 3.7 m) with an 8 ft (2.4 m) ceiling has about 384 sq ft (36 m²) of wall before openings, or roughly 330 sq ft (31 m²) after one door and one window. Two coats is about 660 sq ft of coverage, which is 2 US gallons or 6–7 litres. The ceiling adds 144 sq ft (13 m²), which needs 1 more gallon or about 3 litres for two coats.

How do you calculate wall area for painting?

Add up the length of all the walls to get the room perimeter, multiply by the ceiling height, then subtract the openings — about 1.8 m² (20 sq ft) for a standard door and 1.5 m² (16 sq ft) for an average window. Multiply the result by the number of coats, divide by the coverage on the tin (usually 10–12 m² or 350–400 sq ft per litre or gallon), and add 10% for waste and touch-ups.

How many coats of paint do I need?

Two, in almost every case. One coat looks acceptable while wet and dries patchy, especially in raking light. Plan three coats when you are going from dark to light, when the colour is a saturated red, orange, yellow or deep blue, or when you are painting over a strong stain — or use a primer tinted towards the finish colour, which usually saves the third coat.

What paint sheen should I use on walls?

Eggshell or matt for living rooms and bedrooms, satin for kitchens, bathrooms, hallways and children’s rooms where you will wipe the walls, and flat matt on ceilings. Sheen and durability rise together, but so does how much the light picks up every bump in the wall — the glossier the paint, the more preparation the surface needs.

Do I need primer before painting?

Prime new plaster or plasterboard, bare wood, patched repairs, anything stained by water or smoke, any glossy surface you are painting over, and any big colour change in either direction. You do not need primer to repaint a sound, previously painted wall in a similar colour. On fresh plaster, a mist coat — emulsion thinned roughly 70:30 with water — is the standard first coat.

How long should you wait between coats of paint?

Water-based emulsion is usually recoatable after 2–4 hours; water-based trim paint after about 4–6; oil-based paint needs 16–24 hours. Recoating too early drags the first coat and leaves roller marks that show forever. Paint between roughly 10 and 25 degrees C with humidity under 70%, and give the room airflow rather than heat.