Works out the ventilation a room needs two ways, from its volume and from the number of people in it, and takes the larger. Building codes ask for half an air change an hour in habitable rooms, and holding carbon dioxide down takes about 20 m³ an hour per person.
The ventilation a room needs is worked out two ways, and the larger figure is taken.
is the volume of the room, the air changes per hour and the requirement per person. Codes ask for half an air change an hour in habitable rooms, and holding carbon dioxide below 1000 ppm takes about 20 m³ an hour per person.
The air change rate is about diluting what the building materials give off, so a larger room needs proportionally more. The per-person figure is about diluting the carbon dioxide people breathe out, and has nothing to do with the size of the room. A crowded small room is governed by the second, a sparsely occupied large one by the first.
A room 3.6 by 2.7 m with a ceiling at 2.4 m holds two people. Its volume is 23.3 m³, so half an air change is 11.7 m³/h. The people call for m³/h, which is larger and therefore governs. That works out at 1.7 air changes an hour.
A room of 20 by 10 by 3 m with the same two people reverses it: 300 m³/h from the volume against 40 from the people.
Kitchens, bathrooms and anywhere with open flames or heavy moisture are covered by separate requirements. This calculation is for habitable rooms.