Paint Calculator
How much paint a room actually needs, with doors and windows deducted, so you buy once.
In short
Multiply the room perimeter by the wall height, subtract about 1.8 m² per door and 1.5 m² per window, multiply by the number of coats, then divide by the paint's coverage, usually 10 m² per litre.
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The calculation
- Perimeter = 2 × (length + width)
- Gross wall area = perimeter × wall height
- Subtract openings: roughly 1.8 m² per door, 1.5 m² per window
- Multiply by coats
- Divide by coverage: usually 10 m² per litre for emulsion
A 4 × 4 m room with 2.4 m ceilings, one door and two windows: perimeter 16 m, gross area 38.4 m², less 4.8 m² of openings gives 33.6 m². Two coats is 67.2 m², which at 10 m² per litre is 6.7 litres: so a 5 L tin plus a 2.5 L tin.
Coverage figures on tins are optimistic
Manufacturer numbers assume a smooth, sealed, previously painted surface applied by someone who does this for a living. Reality intrudes:
| Surface | Realistic coverage |
|---|---|
| Previously painted, smooth | 11–13 m²/L |
| Typical painted wall | 10 m²/L |
| Lightly textured | 8–9 m²/L |
| Heavily textured or woodchip | 6–7 m²/L |
| Bare plaster, first coat | 5–7 m²/L |
| Bare wood or MDF | 6–8 m²/L |
Bare plaster is thirsty. It needs a mist coat first, emulsion thinned with about 10–20% water, which soaks in and seals the surface. Skip it and your topcoat will peel off in sheets later.
How many coats, honestly
Two is the answer almost every time. One coat looks patchy in daylight even when it looked fine while wet.
Three if any of these apply:
- Light colour over a dark one
- Strong reds, yellows or oranges. The pigments have poor opacity
- Bright white over a colour
- Covering stains, smoke damage or marker (use a stain-blocking primer first)
Rather than three coats of an expensive colour, a tinted primer close to your final shade is usually cheaper and works better.
What else to buy
- Primer or mist coat for bare surfaces.
- Woodwork paint: skirting, architrave, doors. A 750 ml tin covers the trim in a typical room, twice.
- Ceiling paint if you're doing it, same arithmetic on the floor area.
- Filler and sandpaper. Paint hides nothing; it highlights everything.
- Masking tape, dust sheets, roller sleeves, a brush for cutting in.
Buying tins sensibly
Emulsion comes in 2.5 L and 5 L tins, and the 5 L is nearly always better value per litre. Buy slightly over rather than under. Running out mid-wall means a second trip, and worse, a possible batch difference in colour.
Keep the leftovers. Touch-ups happen, and matching a colour later from memory is hopeless. Write the room name and date on the lid.
Two things that matter more than the paint
Preparation. Filling, sanding and cleaning determine the result far more than which brand you buy. Expensive paint over bad prep looks worse than cheap paint over good prep.
Cutting in. Brush a band around edges and corners first, then roller the middle while the band is still wet so the two blend. Doing it the other way round leaves a visible ridge.
Frequently asked questions
How much paint do I need for a room?
Measure the perimeter, multiply by the wall height to get gross wall area, subtract doors and windows, then divide by the paint's coverage (usually 10–12 m² per litre) and multiply by the number of coats. A typical 4×4 m room with 2.4 m ceilings needs roughly 6–7 litres for two coats.
How many coats do I need?
Two, almost always. One coat rarely gives even colour, and the second is what makes the finish look professional. Three if you are going light over dark, using a strong colour, or painting a red or yellow which have poor opacity. Bare plaster needs a mist coat first, watered-down emulsion. Which does not count as a full coat.
Should I subtract doors and windows?
Yes, but do not be too precise about it. A standard door is about 1.8 m² and an average window 1.5 m². Subtracting them avoids buying a whole extra tin you will not use. Leave the small overage. You want a little left for touch-ups.
Does the ceiling need different paint?
Usually. Ceiling paint is formulated to be flat, to hide imperfections and to drip less. Coverage is similar to wall emulsion, so the same arithmetic applies to the floor area. Add it in below if you are painting it.
Why does the tin say more coverage than I get?
Manufacturer coverage figures assume a smooth, sealed, primed surface and an experienced hand. Textured walls, bare plaster, and rollers all consume more. Bare plaster can absorb 30–50% more on the first coat. Buy for the lower end of the stated range.