Paint Calculator
Paint needed for a room from wall area, coats and coverage, with tins and cost.
About
Paint Calculator
Buying paint goes wrong in both directions. Too little means a second trip and a batch that may not match; too much means half-full tins in the garage for a decade. The calculation is simple enough to be worth doing properly.
The method
- Wall area: 2 × (length + width) × height
- Subtract doors and windows
- Add the ceiling if you are painting it
- Multiply by the number of coats
- Divide by the coverage printed on the tin
A worked example
A room 5 m by 4 m with 2.5 m walls, one door, two windows, two coats, and paint covering 11 m² per litre:
- Wall area: 2 × (5 + 4) × 2.5 = 45.0 m²
- Less one door at 1.8 m² and two windows at 1.2 m² each: 45.0 - 4.2 = 40.8 m²
- Two coats: 40.8 × 2 = 81.6 m²
- Paint: 81.6 ÷ 11 = 7.42 litres
- Tins: 7.42 ÷ 2.5 = 2.97, so three 2.5 litre tins
That leaves 0.58 litres over, which is about right. Some spare is useful for touching up scuffs later, and the leftover from the same batch will match exactly in a way a fresh tin may not.
Coverage is not a fixed number
The figure on the tin assumes a smooth, sealed, previously painted surface. Real walls rarely oblige.
| Surface | Effect on coverage |
|---|---|
| Previously painted, smooth | As stated on the tin |
| New plaster | Reduce by 20% to 30%; it drinks the first coat |
| Textured or heavy woodchip | Reduce by 20% or more |
| Bare wood | Reduce by 15% unless primed |
| Rollered rather than sprayed | Slightly less efficient, more waste in the tray |
New plaster should get a mist coat first, which is emulsion thinned with about 10% water. It soaks in and seals the surface, and it costs far less than wasting a coat of finish paint doing the same job badly.
How many coats
Two is standard and covers most repaints. Three is needed when the colour change is large, particularly light over dark, or when covering a strong colour such as red, which is notoriously difficult because the pigment is transparent enough to keep showing through.
One coat is realistic only when refreshing the same colour on a sound surface. Paint sold as one-coat generally means one coat over a similar shade, and reading it as one coat over anything leads to a patchy finish.
Ceilings, woodwork and the bits people forget
A ceiling is length times width, and it is the same area as the floor. In the example above that is 20 m², close to half the wall area again.
Skirting, architrave, doors and window frames all take a different paint, usually satin or gloss, and are bought separately. Their area is small but the perimeter is not: the room above has 18 m of skirting. One litre of trim paint typically covers the woodwork of two average rooms.
A door takes roughly 0.25 litres for two coats on both sides. Radiators need their own enamel. None of this affects the emulsion calculation and all of it affects the shopping list.
Emulsion, satin and gloss
Matt emulsion hides surface imperfection well and marks easily, which suits ceilings and low-traffic walls. Silk and satin emulsion wipes clean and shows every bump in the plaster. Eggshell and gloss go on wood and metal, with gloss the hardest wearing and the most unforgiving of a poor surface underneath.
Kitchens and bathrooms want a moisture-resistant finish rather than standard matt, since condensation on a matt wall grows mould in the film rather than on top of it.
Buying and storing
Buy every tin at once so they share a batch number, printed on the label. Two tins mixed weeks apart can differ slightly in shade, and the difference shows on a large wall under raking light. Where several tins are needed for one wall, pour them all into a bucket and mix, which is called boxing and removes the problem entirely.
Leftover paint keeps for years if the lid is sealed properly and the tin is stored somewhere that does not freeze. Wipe the rim before closing, and store the tin upside down so the paint itself seals the lid against air.
Estimating a whole house
Doing every room at once is worth the ten minutes, because paint is cheaper per litre in larger tins and a single trip beats five.
| Room | Size | Wall area after openings |
|---|---|---|
| Living room | 5.5 × 4.2 m | 42.4 m² |
| Kitchen | 4.0 × 3.0 m | 29.4 m² |
| Bedroom 1 | 4.0 × 3.5 m | 31.8 m² |
| Bedroom 2 | 3.5 × 3.0 m | 27.0 m² |
| Bathroom | 2.5 × 2.0 m | 17.4 m² |
| Total | 2.4 m ceilings | 148.0 m² |
Two coats over 148 m² at 11 m² per litre needs 26.9 litres, so three 10 litre tubs with a little spare. Buying it as 2.5 litre tins would cost substantially more for the same paint.
What the rest of the job costs
Paint is rarely the largest line. A first-time decorator needs rollers, a frame, an extension pole, brushes, a tray, dust sheets, masking tape, filler, sandpaper and a scraper, which together often come to more than the paint for a single room. Most of it is reusable, so the second room is far cheaper than the first.
Rollers hold paint that never reaches the wall. A medium pile sleeve absorbs a few hundred millilitres that stays in the sleeve and the tray, which is another reason to round the tin count up rather than buying exactly the calculated volume.
Order of work
Ceiling first, then walls, then woodwork. Working downward means splashes land on surfaces not yet finished. Cutting in around edges with a brush before rollering the main area gives a cleaner line than trying to roller into a corner.
Keep a wet edge, meaning always roll back into paint that has not yet dried. Letting a section dry and then painting up to it leaves a visible lap mark, and it is the most common reason a wall looks patchy despite two full coats. This is also why running out of paint halfway across a wall causes a problem no amount of touching up fixes.
Painting over a problem
Paint covers colour and hides almost nothing else. Damp, nicotine staining, water marks and knots in bare wood all bleed back through emulsion, often within days, and repainting the same area achieves nothing because the stain travels through each new coat.
Each has its own sealer. Stain-blocking primer handles water marks and nicotine, knotting solution handles resin bleeding from timber, and damp needs the source fixed before anything is painted at all. Filling and sanding cracks before starting is the other step people skip, and a filled crack that was not sanded flat is visible under any sheen above matt.
Common mistakes
Forgetting the second coat. Coverage figures are per coat, so two coats need double the paint.
Subtracting a flat 10% for openings. Fine for a small window, badly wrong for a patio door.
Using the tin figure on new plaster. Bare plaster absorbs far more than the label assumes.
Buying exactly enough. Round up. Running out mid-wall leaves a visible line where the paint dried before you got back.
Common questions
Frequently asked questions
Most average bedrooms of about 4 m by 5 m need 7 to 8 litres for two coats on the walls, which is three 2.5 litre tins. Add roughly another 4 litres if you are painting the ceiling as well. The exact figure depends on the wall height and the coverage printed on the tin.
Standard emulsion covers 10 to 13 square metres per litre per coat on a smooth painted surface, and a US gallon covers 350 to 400 square feet. New plaster, textured walls and bare wood all reduce it, sometimes by a third.
Two for almost any repaint. Three when going light over dark, or over a strong colour such as red. One coat is realistic only when refreshing the same shade on a sound, sealed surface.
Yes, at their real size. A standard door is about 1.8 square metres and a window about 1.2, so a room with one door and two windows loses over 4 square metres. A patio door or a large picture window changes the answer far more than a rule of thumb allows.
It is emulsion thinned with about 10% water, painted onto new plaster to seal it. Without it the plaster absorbs the first proper coat unevenly and the finish looks patchy. It is cheaper and faster than wasting a coat of finish paint on the same job.
Not successfully. Emulsion has no durability on wood and will chip and mark quickly. Skirting, frames and doors need eggshell, satin or gloss, which is bought separately and calculated by the length of trim rather than by wall area.
Because tinting varies slightly between production batches. Buy all the tins together and check they share a batch number, or pour them into one bucket and mix before starting, which removes any difference.
Several years if the lid is sealed tightly and it is stored somewhere frost-free. Wipe the rim clean before closing so the lid seats properly. Paint that has separated can usually be stirred back; paint that smells sour or has skinned right through should be discarded.