How many cubic metres is my room?
Multiply the floor area by the ceiling height. A room of 15 m² with a 2.5 m ceiling holds 37.5 cubic metres of air — and under a sloped ceiling you use the average height instead.
Short answer: Volume is floor area × ceiling height, given in cubic metres (m³). A room of 4.20 × 3.60 m with a 2.50 m ceiling is 15.12 × 2.50 = 37.80 m³. If the room has a sloped ceiling, you multiply the floor area by the room's average height instead — that sits midway between the knee-wall height and the ridge height. Volume is the number dehumidifiers, air purifiers, heat pumps and ventilation systems are sized against; area alone is not enough.
The formula, and a worked example
Volume is just as simple as area — it only needs one extra measurement:
Volume = length × width × ceiling height
A living room of 4.20 m × 3.60 m with a 2.50 m ceiling:
- Floor area: 4.20 × 3.60 = 15.12 m²
- Volume: 15.12 × 2.50 = 37.80 m³
One cubic metre is a cube 1 × 1 × 1 metre. So the 37.80 m³ is the amount of air in the room — roughly 45 kg of air, if you want it as a weight.
Rooms with a sloped ceiling: use the average height
Under a sloped ceiling there is not one ceiling height, but infinitely many. The way round it is to work out the room's cross-section and multiply by the length — or, which comes to the same thing, to multiply the floor area by the average height.
An attic room is 4.00 m wide and 5.00 m long. The knee wall is 1.10 m high on both sides, and the ridge sits 2.60 m above the floor.
The cross-section is a rectangle with a triangle on top:
- The rectangle: 4.00 × 1.10 = 4.40 m²
- The triangle: 4.00 × 1.50 ÷ 2 = 3.00 m²
- Cross-section total: 7.40 m²
- Volume: 7.40 × 5.00 = 37.00 m³
The check: the average height is 7.40 ÷ 4.00 = 1.85 m, and the floor area is 4.00 × 5.00 = 20.00 m². 20.00 × 1.85 = 37.00 m³. Same figure both ways — so the sum is right.
Notice the shortcut for a gable roof with equal slopes: the average height is the knee-wall height plus half the rise, i.e. 1.10 + (1.50 ÷ 2) = 1.85 m. You do not need to work out the cross-section if the roof is symmetrical.
And look at the comparison: the two examples hold almost exactly the same amount of air — 37.8 against 37.0 m³ — even though the attic room has 20 m² of floor against the living room's 15 m². The sloped ceiling eats up the cubic metres the floor area would otherwise give it.
What volume is used for
- Dehumidifiers and air purifiers. Capacity is given per m³, or m³ per hour, not per m². A dehumidifier rated for "50 m³" suits the living room in the example. Buy by floor area instead, and you will get it wrong in a room with a high ceiling.
- Ventilation and air changes. One air change means the room's entire volume of air has been replaced once. The room's m³ is the base figure the whole calculation rests on.
- Heating. A room with a 3.20 m ceiling has to heat 28% more air than the same floor area with a 2.50 m ceiling. Volume, though, is only one of several factors — insulation, windows and exterior wall area matter at least as much. Sizing a heat source correctly is a job for a heating or ventilation professional, not a rough estimate.
- Insulating a knee wall or attic. Insulation is sold by the m² at a given thickness, but the cavity it has to fill is a volume.
- Moving house. Removal firms quote in m³ of belongings, not m² of home.
Measure internally — the height too
Volume has to be worked out from internal measurements: the floor from wall surface to wall surface, and the height from finished floor to finished ceiling. That is the only measure an ordinary person can check with a tape measure, and it is also the only one that describes the air actually inside the room. A gross area measured around the outside of the house includes the brickwork, and brickwork is not air.
Three things to watch for when measuring the height:
- Measure in two places. In older houses there is often a 2-4 cm difference from corner to corner. Use the average, not the tidiest-looking figure.
- Exposed beams lower the volume slightly. A beam that projects 15 cm down takes up a few cubic metres. For most purposes it can be ignored — for a ventilation system it cannot.
- A dropped ceiling does not count. If a new ceiling has been fitted below the old one, it is the new height that belongs to the room.
Why a tall room feels bigger than the figure suggests
Two rooms of 15 m² can have very different volumes. With a 2.50 m ceiling there is 37.5 m³; with 3.20 m — as in many older townhouses — there is 48.0 m³. That is 28% more air in the same floor area.
That is one of the reasons floor area on its own is a poor description of what a home is like to be in. Square metres tell you how much furniture will fit. Cubic metres tell you something about how the room feels — and what it costs to heat.
Volume and HouseSense
HouseSense measures both the floor area and the wall heights when the home is scanned with the iPhone's LiDAR, or drawn on graph paper. So both figures you need for the volume are already taken care of: the room's internal floor area and its ceiling height. Multiply them together and you have the cubic metres.
Sloped ceilings are built by HouseSense according to simple rules based on the shape of the gables — it is a drawing of the roof, not a measurement of it. If you need a precise volume under a sloped ceiling, measure the knee-wall height and the ridge height in the room yourself and work out the cross-section as above. That is the principle the app works by: the drawing may be dressed up, the numbers may not.