What the number on the tin actually means
Coverage — spreading rate, on some labels — is how many square metres one litre covers in one coat. Manufacturers measure it under controlled conditions: a smooth, sealed, non-absorbent test panel, applied at a specified wet film thickness, at a stated temperature and humidity.
None of those conditions describe your hallway. The figure is not dishonest; it is a comparable benchmark, the way a car's fuel consumption is a comparable benchmark. It tells you this emulsion goes further than that one. It does not tell you how many litres your wall will absorb.
This is why coverage is the single largest source of error in paint estimates, larger than measuring mistakes. Get the area wrong by 5% and you are fine. Assume 12 m²/L on new plaster that delivers 7 and you are a third short.
Porosity: the main variable
A porous surface has open capillaries that pull the liquid part of the paint inwards. Less binder and pigment stay on the surface, so the film is thinner, so the colour is weaker, so you apply more. Porosity is why new plaster, old lime render and filler repairs behave so differently from a wall that has been painted three times already.
There is a simple field test. Flick a few drops of water at the wall. If they sit as beads, the surface is sealed and coverage will be close to the label. If they darken and vanish within seconds, the surface is thirsty and you should expect to lose 20-40% of the stated coverage on the first coat.
Uneven porosity is worse than uniform porosity, and it is what you get after filling. The filled patches drink, the surrounding paint does not, and the result is visible as flat dull islands in a satin wall even after the colour matches. The fix is not more coats of finish paint — it is sealing the difference first.
Texture and geometry
A textured surface has more actual area than its measured area. You measure a flat rectangle; the paint has to coat every peak and trough inside it. The rougher the profile, the wider the gap between the two.
| Surface | Effect on coverage | Realistic figure |
|---|---|---|
| Smooth previously painted plaster | Baseline | 10-12 m²/L |
| Skimmed new plaster, unsealed | Heavy absorption on coat one | 6-8 m²/L first coat, then near baseline |
| Plasterboard, unsealed | Paper face absorbs unevenly | 7-9 m²/L first coat |
| Lightly textured or roll-on finish | More real area | 8-10 m²/L |
| Heavy texture, artex, woodchip | Much more real area | 5-7 m²/L |
| Bare or rendered masonry outside | Porous and rough together | 4-6 m²/L |
| Previously gloss-painted woodwork | Non-absorbent but needs a key | 12-14 m²/L, adhesion is the issue |
These are planning figures, not promises. Where your surface sits between two rows, take the lower one: being a litre over costs a few euros, being a litre short costs a trip and possibly a batch mismatch.
Colour and hiding power
Opacity comes from pigment, and pigments differ enormously. Titanium dioxide, the white workhorse, hides brilliantly. Organic reds and yellows hide badly — a bright yellow over white can need three coats, and over a dark base four, no matter how expensive the tin.
That is a coat-count problem rather than a coverage problem, but it hits your shopping list the same way, and it multiplies: an extra coat on a room costs you the whole area again, not a percentage of it.
The professional answer is a tinted base coat. Ask for the undercoat or first coat to be tinted towards the final colour, and a colour that would have needed four coats often needs two plus the base. It costs less than the coats it saves and it looks better, because the final film is thinner and more even.
How you apply it
- Roller, medium pile
- The reference method for the stated figure on walls. A longer pile holds more paint and lays down a thicker film, which is what you want on texture and what costs you coverage on smooth plaster.
- Brush
- Cutting-in uses noticeably more paint per square metre than rolling, which is part of why a 10% allowance exists. In a room with many windows, or panelled doors, this is not a rounding error.
- Airless spray
- Fast and even, but overspray and priming the hose lose material. Add 15-25% over the rolled figure unless you spray regularly.
- Thinning
- The first coat on porous plaster is often thinned 10% with water — a mist coat. It goes further per litre but delivers less film, so treat it as a sealing coat, not as one of your two finish coats.
Choosing a figure before you buy
- Start from the number printed on the product you have actually chosen, not from a general rule.
- If it is a range, use the lower end.
- Do the water test. If the wall drinks, budget the first coat at 60-75% of the stated figure and the second at close to it.
- Add a coat, not a percentage, when the colour is changing significantly.
- Calculate primer separately: it has its own coverage, usually different from the finish.
- For anything textured, drop a band in the table above and check after the first wall — one wall painted tells you more than any datasheet.
Coverage figures in this guide are planning ranges compiled from typical manufacturer datasheets for interior emulsion. Always prefer the figure printed on your own tin, and treat any single number as a starting point to be corrected after the first coat.