Room area and volume

What it's for

Handy to estimate paint, flooring, heating and air conditioning. The volume (m³) is often needed to size stoves and AC units.

Note

The wall surface does not remove doors and windows: subtract them separately if you need a precise value.

What volume is for, and why so many people ask for it

Area is enough for the floor and for paint, but almost everything to do with air wants volume: sizing a stove or an air conditioner, the air change rate of a mechanical ventilation system, an estimate of heating demand, the capacity of a dehumidifier. It is why the first question in a shop is «how many cubic metres?» and not «how many square metres?».

In Italian building rules, moreover, the minimum habitable height is 2.70 metres for living rooms and 2.40 for bathrooms and corridors, with exceptions in mountain municipalities and in loft conversions: those are the two figures that make it worth measuring the real height before planning any work.

Rooms are not boxes

Length times width times height is exact only in a rectangular room with a flat ceiling. In reality there are recesses, projections, flues and pillars, and the simple way through is to divide the room into rectangles and add them up, rather than looking for a single formula.

The common case is the sloping ceiling of a loft: there the volume is the floor area times the average height, that is the mean between the lowest and the highest point, which is exact for a single regular slope. For liveability, though, what counts is not volume but how much floor sits under a sufficient height, and that is what building rules ask about.

Measuring properly, in three moves

Measure at floor level and not in mid-air: a tape held in the air sags, and in a five-metre room the error reaches a centimetre. Measure at two opposite points, because almost no wall is parallel to the one facing it, and in old houses the difference between the two sides can exceed five centimetres: use the smaller figure for anything that has to fit (a wardrobe, a rug) and the larger one for material to buy.

And measure the finished surfaces, that is what is there today: drawings are often dimensioned to the bare structure, and between the two there are three or four centimetres per side.

Openings, and when to subtract them

The wall area that comes out here includes doors and windows, and that is the right starting point. For painting, large openings are subtracted and small ones are not (around each one there is an edge to cut in with a brush, which uses more). For tiling a bathroom they are subtracted, and the reveals, that is the side faces of the openings, are added instead.

For volume, on the other hand, openings are irrelevant: the air in the room is what it is, with the door open or shut.

Nearby tools

From here start How much paint you need, How many tiles you need and How much wallpaper you need. Volume feeds Air conditioner BTU, area feeds How many lumens you need. For non-rectangular shapes there is Areas and perimeters.