Paint is sold by volume and labelled with a coverage rate — "10 m² per litre" is typical. The number is real but optimistic, and the gap between it and your actual wall is where most paint projects run short.
The calculation, and then the correction
Start with the geometry:
wall area = perimeter × height − openings
A 4.2 × 3.6 m room with a 2.5 m ceiling gives a perimeter of 15.6 m, so 39 m² of wall. Subtract a 3.6 m² door and a 1.68 m² window and you have 33.7 m² to paint. Add a ceiling if you are painting it: 15.12 m², bringing the total to 48.8 m².
Then coats and coverage:
litres = area × coats ÷ coverage
Two coats at the label rate of 10 m²/L gives 48.8 × 2 ÷ 10 = 9.8 litres. And that is where the arithmetic stops being useful.
Why the label rate is optimistic
The 10 m² figure is measured under conditions that rarely exist in a home:
- On smooth, sealed, primed surfaces. New plaster is porous and drinks the first coat — coverage can drop to 6-7 m²/L. Fresh gypsum needs a primer for exactly this reason.
- With a roller, not a spray gun. A roller wastes roughly 10% on the tray, the roller nap and the sides of the tin.
- On one coat over dry, previously painted. The label rate is a one-coat figure over a sound surface. A second coat always covers less because the surface is no longer absorbing.
- At a specific temperature and humidity. Cold, slow-drying paint spreads thinner and covers less.
Realistic planning figures, taken as a fraction of the label rate: new plaster 0.6-0.7×, textured surface 0.7-0.8×, previously painted in good condition 1.0×, dark over light with two coats 0.8×.
Three things that change how much you need
Colour. Changing from a light to a dark colour, or vice versa, needs a full extra coat and a compatible primer. A dark colour over a light one cannot be achieved in one coat at any price.
Surface. Texture is the biggest single variable. A heavily textured ceiling can absorb half again as much as a flat one, because the roller has to fill the valleys as well as cover the surface. If the label assumes smooth, textured means dividing by 1.5.
Number of coats. New plaster, bare timber, a colour change or a repair all need primer plus two coats — effectively three passes. The primer is cheap and it is the difference between a coat that adheres and one that peels off in a year.
Buying and storing
Paint is sold in 1 L, 2.5 L and 5 L tins, and the price per litre falls sharply with size — a 5 L tin is commonly 30-40% cheaper per litre than two 2.5 L tins. The catch is that a part-used tin cannot be stored indefinitely: latex paint keeps for a couple of years sealed, but oil-based and alkyd paints are more sensitive to temperature and go off sooner.
The practical strategy is to buy the largest size that keeps the waste under about 10% of what you use. For 9.8 litres of need, a 10 L (two 5 L) purchase is 2% waste and clearly right.
Coverage, not drying
Paint feels dry long before it is cured, and this matters for the next coat. Typical latex: 1-2 hours to tack-free, 4 hours to recoat, 2-4 weeks to full cure. Full cure is when the paint reaches its final hardness and colourfastness. Applying the second coat too soon — before the first is recoatable — is what makes a coat lift, wrinkle or stick to itself, and the repair is a full strip back to bare.
A practical method that avoids a second trip
- Measure properly: perimeter × height, minus doors and windows, plus the ceiling if included.
- Decide the coats first: two over sound paint, three over bare plaster, timber, or a colour change.
- Apply the label rate, then multiply by a surface factor — 0.65 for new plaster, 0.75 for textured, 1.0 for good existing paint.
- Round up to the next tin size and accept that leftover paint is normal and storable.
- Remember the roller when estimating: budget 10% extra for the tray and nap, and keep a sealed foil or plastic-wrapped brush for the touch-ups that always come.
The primer is not optional on most surfaces
Primer exists for three separate reasons, and it is worth knowing which one applies because they have different costs.
Sealing. New plaster, bare timber, gypsum board joint compound and MDF are porous and will absorb the first topcoat unevenly. A sealing primer equalises the absorption so the finish coat goes on evenly. Skipping it on new plaster is the most common cause of patchy walls, and no amount of careful rolling fixes it — the difference was set in the first five minutes.
Adhesion. A glossy or non-porous surface has nothing for paint to grip. Painting over gloss without abrading and priming gives a finish that looks fine and peels off in sheets a year later. The test: if a fingernail can scratch it, it needs sanding and priming.
Colour blocking. A stain-blocking primer over a water mark, a nicotine mark or a dark colour prevents it from teleporting through the finish coat. Two coats of ordinary paint over a stain will still show it.
How coats compound, and why the second one is thinner
A two-coat finish is not two independent layers of the full thickness. The first coat does most of the filling and is typically applied generously; the second is thinner and exists mainly to even out the sheen and hide the first coat's marks. This is why:
- The first coat should look wrong when wet and slightly patchy when dry. That is normal, and it is what the second coat is for.
- Rolling too thin on the first coat is a mistake, and the temptation to fix it with a heavier second coat causes drips and sag.
- Total film thickness matters more than coat count. Three thin coats outperform two thick ones, because thick paint on a wall does not cure evenly — the skin hardens while the interior stays soft and eventually cracks.
Choosing between a cheap and an expensive paint
The coverage figure on the tin is the single best guide to quality in cheap paint, and it is more reliable than any marketing claim. Paint formulated for lower-quality surfaces uses more binder and more pigment solids, and those are exactly the two things that determine whether a litre covers 16 m² or 9 m². If two tins claim the same area, the more expensive one usually is doing something with the resin.
Where premium paint genuinely earns its price: a scrub-resistant alkyd in a high-traffic hallway, a low-VOC formulation in a bedroom, and a self-leveling enamel on furniture. For a bedroom wall, mid-range paint is indistinguishable from premium once it is on, and the difference is measurable in coverage.
Ventilation, temperature and the drying clock
Paint dries by evaporation of the carrier, and everything that slows evaporation stretches the working time and increases the chance of problems:
- Below about 10 °C, latex paint will not cure properly and may never become durable, regardless of how long you leave it. This is a hard limit, not a guideline.
- High humidity slows drying further — which is why painting in a damp season or in a bathroom with no window open takes twice as long.
- Good airflow is the free accelerator. Windows and a fan shorten drying dramatically and also help you notice the smell sooner.
The reason full cure takes 2-4 weeks is that it is a polymerisation process continuing at a decreasing rate. Until it completes, the film is still slightly soft, so heavy furniture placed against a wall in the first week can stick and take paint with it when moved. Leaving a gap for a month is a real, practical precaution that costs nothing.
Frequently asked questions
What is the difference between primer and paint?
Primer does three jobs: it seals porous surfaces so the finish coat applies evenly, it provides adhesion on non-porous surfaces that would otherwise peel, and it blocks stains so they do not come through. On new plaster, bare timber, gloss or a stained wall it is not optional.
How many coats of paint should I apply?
Two over sound previously-painted walls. Three (including primer) over bare plaster, new timber, a colour change, or a repair. One coat is only ever enough for a quick refresh of a recently painted wall in the same colour, and it will show every joint and patch.
Can I paint over gloss without sanding?
You can, but you should not expect it to last. Gloss has nothing for paint to grip, so abrade it and use an adhesion primer. If you skip both, the finish may look acceptable and then lift off in sheets within a year, taking the undercoat with it.
Why does my paint look patchy after the first coat?
That is expected, and it is what the second coat is for. The first coat fills and seals, and the second evens out the sheen. What is not expected is patchiness after two coats, which means the surface was too porous or too different in absorption — seal it with primer and redo it.
Frequently asked questions
How much paint do I need for a room?
Measure the paintable wall area (perimeter x height, minus doors and windows), multiply by the number of coats, then divide by the coverage rate. A 33.7 m² wall painted twice at 10 m² per litre needs 6.7 litres, so buy 7-8 to allow for the roller losses the label figure ignores.
Why does the tin say 10 m² per litre when I get less?
The label figure is measured on a smooth, sealed, freshly rolled surface in ideal conditions. Real walls absorb the first coat, textured walls absorb more, and a roller leaves roughly 10% in the tray and on the roller. On new plaster or a textured surface expect 30-40% less coverage than the label.
Do I really need two coats?
Almost always, and the primer is not optional on new plaster, bare timber or a colour change. One coat shows every joint and patch, and a dark colour over a light one needs a full second coat to reach the intended depth. If you skip the second coat, budget for doing it later.
How long does paint take to dry?
Tack-free after 1-2 hours, recoatable after 4 hours, fully cured after 2-4 weeks. The 'dry to the touch' feeling is not 'dry' — full cure takes weeks, which is why a heavy coat applied too early can lift or stick to itself.