You buy a 5-litre tin of paint. The label says it covers "up to 50 square meters." Your wall is exactly 40 square meters. You do the math, feel confident, and start rolling. Halfway through the second coat, you run out.
If this has happened to you, you are not alone. The coverage numbers printed on the side of paint tins are not lies, but they are also not realistic for most construction sites. They are calculated under ideal, laboratory conditions that almost never exist in the real world.
01. The Manufacturer's Math
When a paint manufacturer calculates coverage (usually expressed as square meters per litre, or square feet per gallon), they do it on a perfectly flat, non-porous, uniform test panel.
They apply the paint at a very specific, controlled wet-film thickness. This thickness is the bare minimum required to achieve the stated colour and opacity. In the lab, they do not have to deal with textured drywall, porous masonry, or a painter who is rolling it on slightly thicker to "make sure it covers."
In reality, the number on the tin represents the absolute maximum theoretical coverage. You should always treat it as a "best case scenario," never as the expected outcome.
02. Factor 1: Surface Porosity
The single biggest reason paint runs out early is surface porosity. The paint is absorbed by the surface before it has a chance to form a film on top.
The First Coat is Always Sacrificial
On new drywall, bare plaster, or raw wood, the first coat of paint acts like a sponge. It is absorbed by the material. You will almost always use 30% to 50% more paint on the first coat than on the second.
This is why using a primer is critical. A high-quality primer seals the pores of the drywall or wood. While you pay for the primer upfront, it usually saves you money overall because your expensive topcoat paint sits on the surface instead of soaking into the wall.
Textured surfaces (like stucco or heavy orange-peel drywall) also increase surface area. A wall with heavy texture can have 20% more physical surface area than a smooth wall, requiring more paint to cover the peaks and valleys.