Buying too much paint leaves open tins unused; buying too little forces another trip to the shop in the middle of the job, with the risk that the new tin doesn't match the previous one exactly in tone. The right way to avoid it is to calculate by coverage, not by eye.
The basic calculation: coverage per litre
Quality matt emulsion covers on average 12 m² per litre per coat on a smooth wall. On rough or very porous walls that coverage falls to 8–10 m²/litre, so it's wise to be conservative if the wall isn't well prepared.
The formula is simple:
Litres needed = (Area × Number of coats) ÷ Coverage
Example: to paint 40 m² of wall with 2 coats of matt emulsion: (40 × 2) ÷ 12 = 6.7 litres, rounded up to 2 tins of 5 litres. This is the same calculation the PresupuestoYa calculator applies automatically for the Painter trade.
Coverage changes with the finish
Not all finishes go equally far, and mixing them up throws the calculation off quite a bit:
| Finish | Coverage per coat | Where it's used |
|---|---|---|
| Matt emulsion | 12 m²/l | Living rooms, bedrooms |
| Satin emulsion | 10.5 m²/l | Kitchens, bathrooms, corridors (more washable) |
| Anti-damp/anti-mould | 11 m²/l | Areas with recurring damp stains |
| Exterior facade | 9 m²/l | Exterior, in 15 L tins |
| Enamel (joinery/metal) | 10 m²/l | Doors, windows, radiators, in 0.75 L tins |
Satin cleans better and resists occasional damp, so it is the recommended option in kitchens, bathrooms and high-traffic areas, although it shows wall imperfections slightly more than matt.
What to add before the paint
The tin of paint isn't the only item:
- Primer/sealer: mandatory on new plaster, very porous or stained walls, covers 10 m²/litre. If the wall is already painted in good condition, you can save it.
- Filler: for cracks and holes before painting, 1 kg covers 2–3 m² in a thin application; if there are many cracks, allow 1.5 times more.
- Smoothing textured coating: if old texture has to be removed, allow 1.2 kg of levelling compound per m², plus 2–3 coats and careful sanding between coats — it's one of the jobs that takes the most real time in a painting renovation.
Conditions that affect the result
Don't apply paint below 10°C or with humidity above 80%: drying will be poor and the paint can end up flaking. Also respect the drying time between coats: minimum 4 hours for emulsion, 24 hours for enamel. Applying over a coat that's still wet ruins the final finish.
Frequently asked questions
How much paint do I need to paint a complete flat with 80 m² of wall? With 2 coats of matt emulsion: (80 × 2) ÷ 12 ≈ 13.3 litres, i.e. 3 tins of 5 litres.
Is it better to buy more or just enough? More: it's preferable to have a spare tin for future touch-ups than to run short and not find exactly the same tone later.
In what order is a room painted? First the ceiling, then the walls and finally skirting and joinery, with the floor protected from the start.
Conclusion
Calculating by coverage instead of "by eye" is what separates a tight painting quote from one that falls short at the second tin or leaves unused material. Automating it, including primer and filler when needed, gives a clear, professional breakdown for the client.
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