Various units are used to describe the rate of ventilation. These include:

**Volumetric flow rate:** Ventilation and air infiltration is commonly expressed in terms of a volumetric air flow rate e.g. litres/s (l/s) or m³/s.

**Per occupant air flow rate:** Sometimes the volumetric flow rate is divided by the number of occupants in a space to give a flow rate for each occupant. This is commonly expressed in terms of litres/second for each occupant, i.e. l/s.p.

**Unit area flow rate:** Alternatively, the air flow rate may be divided by the floor area of an enclosure to give a unit area value, i.e. litres/second.m².

**Air change rate:** Air flow is also often expressed in terms of hourly ‘air change rate’ (ach). This is the volume flow rate of air into an enclosure (e.g. a room or the entire building) divided by the room (or building) volume.

**Mass flow rate:** Sometimes air flow rate is expressed in terms of the mass flow rate of air, e.g. kg/s. Mass flow is needed to determine the thermal energy carried by the air stream. It is also widely used in ventilation and air flow calculation techniques.