Paint tins state a coverage area that most decorating jobs fail to achieve. The figure is accurate under the conditions it was measured in, which are rarely the conditions on site.
How the figure is produced
Coverage is measured on a smooth, sealed, uniform surface, applied at a controlled thickness under stable temperature and humidity by someone doing nothing else.
Those conditions maximise spread, because none of the paint is absorbed into the substrate and none of it is lost to texture or to uneven application.
The number is therefore a best case rather than an average, and it is quoted per coat, which is easy to overlook when estimating for a whole room.
What the surface takes
Porous surfaces absorb paint rather than holding it on top, and fresh plaster, filler, bare timber and previously unpainted board all absorb a great deal.
Texture increases the actual area being covered, so a lightly textured wall consumes noticeably more than a smooth one of exactly the same measured dimensions.
This is why a sealing or priming coat is specified. It converts an absorbent surface into a uniform one so the following coats behave as the figure assumes.
Why colour changes the quantity
Opacity varies with the pigments used, and certain colours are inherently poor at obscuring what lies beneath them regardless of how carefully the paint is applied.
Strong reds, yellows and oranges are the usual cases, because the pigments producing those shades cover far less effectively than the white pigment used elsewhere.
Large changes in tone require more coats as well, since covering a dark wall with a light colour is a different task from refreshing an existing shade.
Where the rest of the tin goes
Application method matters. Rollers hold paint within the nap, brushes hold it in the bristles, and sprayers lose a portion to overspray that never reaches the wall.
Cutting in around edges, sockets and frames uses more per square metre than open wall, because the work is slower and the coat tends to go on thicker.
Waste in trays, on tools and in cleaning accounts for a further share, and none of it appears in a figure measured under laboratory conditions.
Estimating with a margin
Calculating wall area, dividing by the stated coverage and then adding a margin for absorption, texture and extra coats is far closer to what a job actually needs.
Buying the whole quantity at once also matters, because tinted paint can vary slightly between batches and that difference shows on a large continuous surface.
Keeping the remainder sealed and labelled is worth more than the paint itself, since matching an existing colour later is considerably harder than it appears.