Sheen is one decision that controls two things
Sheen is how much light a dried paint film bounces back at you rather than scattering. That sounds like a purely visual choice, and it is often sold as one, but it quietly decides two practical properties at the same time, and they pull in opposite directions.
The first is durability. Higher sheen generally means a harder, tighter, more tightly bound film, because the resin content is higher relative to the pigment and filler. That film resists scrubbing, wipes clean, and shrugs off the odd wet hand. A flat matte film is more porous by comparison, and washing it tends to burnish a shiny patch into the surface rather than clean it.
The second is how honest the paint is about the wall underneath. A flat finish scatters light in every direction, so a slightly wavy plaster skim, a patched crack or a proud joint simply disappears. A gloss finish reflects light directionally, which means every ripple, every filler edge and every sanding scratch is drawn out in relief across the wall. The shinier the paint, the better the surface preparation has to be.
Almost every sheen decision is a trade between those two. You want washable where things get touched, and you want forgiving where the substrate is imperfect.
The ladder, from flat to shiny
Manufacturers name these differently and measure them differently, so treat the order as reliable and the names as approximate. The number that matters is on the data sheet, not in the marketing.
- Flat or matte: the least reflective and the most forgiving. Best on ceilings and on large wall areas where the plaster is not perfect. Poor at being washed, so keep it away from anything that gets handled.
- Matt emulsion with a wipeable claim: a modern middle ground. Still visually flat, but formulated to survive a damp cloth. The usual choice for living rooms and bedrooms on walls you would rather not have to repaint often.
- Eggshell: a low, soft sheen. Durable enough for hallways, stairwells and children's rooms, and still forgiving enough for ordinary plasterwork. On woodwork it gives a modern, low-key finish.
- Satin: a clear but soft shine. Common on interior woodwork, doors and trim, and on kitchen and utility walls. Shows surface defects noticeably more than eggshell.
- Semi-gloss and gloss: the hardest and most washable, and the least forgiving of all. Traditionally for skirtings, architraves, door frames, and anything that takes constant handling or moisture.
Choosing by room
- Ceilings: flat, almost always. A ceiling is lit at a grazing angle by daylight from a window, which is the worst possible light for showing up an uneven surface, and nobody scrubs a ceiling.
- Bedrooms and living rooms: matte or wipeable matte. Little handling, large uninterrupted wall areas, and the quality of light matters more than washability.
- Hallways, stairs and landings: eggshell. This is the highest-traffic wall surface in a house and the one most likely to be marked by bags, coats and shoulders.
- Kitchens and bathrooms: eggshell or satin, in a formulation intended for moisture. The point is a film that resists condensation and cleaning, not simply a shinier one.
- Woodwork and trim: satin or gloss. Trim is small in area, gets touched constantly, and is made of a material you can actually sand flat, so the extra sheen costs you nothing in forgiveness.
What sheen does not change
It does not change the geometry of your estimate. The area of the walls, the openings you subtract and the number of coats are the same whichever sheen you pick, and the coverage rates used by the paint calculator on this site are for standard interior emulsion.
What it can change is how much preparation the job needs before any paint goes on. Choosing gloss over matte does not add litres to the order, but it may well add a filling and sanding day that a flat finish would have hidden. That is the cost people forget to price.
Two more things worth knowing. Sheen levels do not blend, so touching in a small repair on a satin wall almost always leaves a visible patch — plan to paint the full wall corner to corner. And sheen appears to increase with darker colours, because a deep base carries more colourant and less white pigment, so the same nominal eggshell will look shinier in charcoal than in cream.