Paint calculator
Every figure here treats a wall as a rectangle, and paint does not. Woodchip, textured coatings, heavy relief paper and bare brick all carry far more real surface than the rectangle you measured, and coverage falls away accordingly: brickwork can take twice what smooth plaster does. Cutting in is the other quiet cost, because a brush lays paint on thicker than a roller does, so a room chopped up by beams, alcoves and window reveals uses more per square metre than an empty one of the same size.
Paint needed is the paintable wall area times the number of coats, divided by the coverage rate. At the 11 m² per litre emulsion manages on a sealed wall, a 4 × 3 m room with 2.4 m walls leaves 30.36 m² once a door and a window are deducted, which comes to 5.52 litres for two coats: three 2.5 litre tins.
How to work out paint
The number printed on a tin is a manufacturer's spreading rate, measured on a smooth, sealed, previously painted surface under laboratory conditions. It describes the paint rather than your wall. New plaster, fresh filler, old distemper and anything sanded back are porous, and a porous surface takes the first coat into itself instead of leaving it on top. Six to eight square metres a litre is realistic for a first coat on bare plaster against a printed eleven. A mist coat closes that gap: emulsion thinned with ten to twenty per cent water, rolled on to seal the surface, after which every coat lands on the figure the tin promised.
Coats and coverage are separate problems and people run them together. Coverage is how far a litre goes. Opacity is whether you can still see what was underneath, and it depends on how much white pigment the paint carries. A deep colour mixed from a tinting base hides poorly for exactly that reason: the base has had pigment taken out to leave room for the colourant. Going markedly lighter is a three-coat job in any paint, and the cheap way through it is a tinted undercoat in a shade near the finish, which costs less per litre than topcoat and does the hiding in one pass.
The deductions here are fixed sizes rather than measurements. A door comes off at 1.8 m², which is 0.9 by 2.0 metres, and a window at 1.44, which is 1.2 by 1.2. Close enough for a bedroom, badly wrong for a wall that is mostly glass. A set of patio doors is nearer 4 m², so enter it as two or three doors; a bay of three sashes is three windows and not one. The error only ever runs one way, and it runs in your favour, since under-deducting buys paint you did not strictly need.
Rounding to whole tins is where the arithmetic stops being tidy. Those 5.52 litres become three 2.5 litre tins, so 7.5 litres, because nobody sells a third of a tin. In 5 litre tins the same job takes two, which is 10 litres and a worse outcome. Check both sizes before you buy, particularly on a small job. Then decant every tin of a machine-tinted colour into one bucket and stir before you start: tinting happens tin by tin from a base and a colourant dispenser, and two tins of the same code mixed on different days are not guaranteed to match. Decorators call it boxing and it takes two minutes.
What people use it for
- Buying paint for a bedroom, living room or hallway
- Costing a whole-house repaint one room at a time
- Deciding between 2.5 and 5 litre tins for the same job
- Working out the mist coat for a freshly plastered room
- Estimating a single feature wall on its own
- Checking a decorator’s materials line against your own measurements
- Converting a US gallon estimate into litres and tins
- Seeing how much will be left over for touch-ups
Questions
A 12 × 12 ft room with 8 ft walls leaves about 349 ft², or 32.4 m², after one door and one window. That is 2.95 litres per coat and 5.9 litres for two, so three 2.5 litre tins.
With 2.4 m walls and one door and one window it is 35.2 m² of wall, 6.39 litres for two coats. Again three 2.5 litre tins, with about a litre spare.
Two on a similar colour. Three when going markedly lighter, or over a strong colour, or on to a patch-repaired wall where the filler shows through.
Emulsion thinned with ten to twenty per cent water, applied to bare plaster so it seals rather than soaks. On new plaster it is not optional: paint applied neat to raw plaster can peel away in sheets later.
10 to 12 m² for emulsion on a sealed wall, 6 to 8 m² for a first coat on bare plaster, around 16 m² for a thin coat of gloss on woodwork, and as little as 4 to 6 m² for masonry paint on textured render.
At fixed sizes, not measured ones: 1.8 m² per door and 1.44 m² per window. Enter large glazing as several units rather than one.
No. This counts wall area only. A ceiling is simply length times width, needs two coats like a wall, and is nearly always underestimated because it looks smaller than it is.
Because tins round up. 5.52 litres cannot be bought as 5.52 litres, and the leftover is the price of that. Comparing tin sizes usually shaves it.
Barely different. Makers usually quote matt a shade further per litre, but the gap sits inside the variation you should already be allowing for. Choose the sheen for how it wears and how it hides roller marks.
Far less than the area suggests, because gloss and satinwood spread to about 16 m² per litre. The cost is in passes rather than volume: primer, undercoat and topcoat is three trips round the same skirting.
Pouring every tin of one colour into a single bucket and stirring before you begin, so any variation between tins is spread across the whole room instead of appearing as a line halfway up a wall.
Keep whatever the rounding leaves you, sealed and labelled with the room. Touch-ups are certain, and matching a colour two years later against a faded wall rarely works even with the code.
The arithmetic holds, the coverage figure does not. Drop it to 4 to 6 m² per litre on textured render or pebbledash, and 6 to 8 on smooth render, then add a coat if the surface has never been painted.