Guide calculation of air volume


  • Calculate air volume by using air change rate and example
  • Ventilation in case of dry, used and humid air
  • Performance in item descriptions
  • How does a fan move air?  

Calculate air volume by using air change rate and example of use

The calculation of the required air volume requires the volume of the room, which derives from the formula room volume = length(m) x width(m) x height(m) and the purpose of the room that indicates the air change rate (ACH).

The air change rate is often predetermined by legal guidelines and corresponds to measured experience. The data in the chart on the right indicates how often the air should be exchanged in a room within one hour.

The required air mass per hour is then calculated by multiplying this value with the volume of the room. The data depicts “from…to“ values, for example showrooms are shown to require four to eight changes per hours. It comes down to your personal evaluation if you want to orient yourself on the lower or upper recommended value. Say, you sell brand-new textiles that are still drying – in this case it would make sense to calculate with a higher rate.

This calculation of air volume is just an estimate of the demand, DIN standards set specific requirements. 

Example of use:

A classroom measuring 8 metres in length, 5 metres in width and 3.5 metres in height shall be equipped with a new ventilation system. The room is usually attended by 25 pupils. However, as the rooms is frequently used by bigger classes as well and the windows need to stay closed for safety reasons, the air change rate 7 (see chart) can be assumed. The classroom with a volume of 140 m3 thus requires an air volume of 980 m3/h.

  • In case rooms are marked with *ex you need to check if EX-protected fans are required.
  • Apart from the calculation of the ideal air change by using ACH, some cases require rates of fresh air supply per person as well (DIN 19646/2).
  • Sources/guidelines: 1) Empirical values from 2) DIN 1946/2, 3) VDI 2082, 4) DIN 18017, 5) ASR, 6) VDI 2089, 7) VDI 2052, 8) VDI 2051, 9) EN 13779

Now compare the calculated value with the indicated performance of our fans. On top of sufficient airflow noise level and power consumption of the models should be considered as well.  

Type of room



5 – 7

Battery room *ex 1

5 – 10

Pickling plant 1

5 – 15

Conference room 1

5 – 8

Library 1

4 – 5

Office 2

4 – 8

Shower 5

15 – 25

Dye factory 1

5 – 15

Paint spraying room *ex 1

25 – 50

Photo copier room 1

10 – 15

Wardrobes 3

4 – 6

Restaurant (excl. smoking lounge) 2/3

6 – 8

Lecture hall 1

6 – 8

Kitchen, domestic 1

15 – 25

Kitchen, commercial 7

15 – 30

Laboratory *ex 8

8 – 15

Assembly hall 1

4 – 8

Smoking lounge 9

Up to 20

Classroom 1

5 – 8

Indoor swimming pool 6

3 – 4

Boardroom 1

6 – 8

Gym 1

4 – 8

Theatre, cinema 1

5 – 8

Toilet, domestic 4

5 – 8

Toilet, public 1

5 – 15

Sports hall 2

4 – 6

Changing room 5

6 – 8

Waiting room 1

4 – 6

Salesroom 3

4 – 8

Workshops with little deterioration of air quality 3

4 – 6

Workshops with severe deterioration of air quality 3

10 – 20

Living space 1

3 – 6

Ventilation in case of dry, used and humid air

Upon inspecting the air change rate you might notice that damp rooms with showers or stoves require considerably more air changes per hour than e.g. a bedroom. An important factor for the ACH is also how often the room is frequented: the more people are staying in the room, the more oxygen is need. Not only us humans produce carbon monoxide by breathing, pets and even plants contribute as well. You want more information? Read our blog entry "How much oxygen does a human need"

Starting from a CO2 content of 0.1 percent in the breathing air negatively impacts our mental productivity and wellbeing. That is why it is always best to air out the room before addressing yourself to a task that requires concentration, or even better leave ventilation to a fan that automatically knows what to do. The latter can also be equipped with fine particle filters and heat reservoirs for heat recovery.

On the other hand, if the air is saturated with water vapour for longer periods, the humidity settles on absorptive matter, such as wallpapers, wood boarding, furniture, textiles and house wall. Due to condensation gas turns into water on smooth surfaces, unnoticed it quickly becomes a breeding ground for germs. A long lasting relative humidity above 60 % increases risks of mould growth. The spores of this obnoxious infestation are a natural part of the atmosphere and therefore of the indoor air, too. Once they find a suitable breeding ground, they spread quickly and can cause serious diseases such as asthma.

However even smaller water contents in the air can affect physical health, leading to irritation of the mucous membrane and a susceptibility to a cold. Particularly during the period between autumn and spring - when we tend to heat more - humidity levels in the air can drop below 40 %. We perceive these humidity levels as “dry air”. 40 – 60 % are a value you should opt for as they are best to keep humans and building material in a good condition.

Ensuring an ideal climate in enclosed rooms requires a bit of finesse and above all consistent ventilation in appropriate intervals. In addition to the enormous effort spent on manual ventilation (windows, doors), some buildings do not even allow ventilation, because of heat insulation and enclosed rooms without windows. Exposure to noise and allergies can also be a reason to keep the windows closed most of the time. In-line, wall and window fans are a great electrical solution that saves space and most importantly regulates ventilation whenever it is necessary. State-of-the-art models can be connected to light switches, toilet flush or humidity sensors and begin exchanging the air without needing to be switched on manually. This saves precious time and makes it possible to ensure an appropriate exchange even if you are not at home. Sensors also come in handy in buildings that are not frequented as much, e.g. storage rooms.      

Performance in item descriptions

The performance is stated in m3/h in the technical details of the item description. It refers to the airflow achieved on the highest rotation speed level. Seeing that the manufacturer measures this data under ideal airflow conditions, the value can differ from reality. This difference is caused by fluctuation of temperature, dust and other “real-life”obstacles the airflow needs to overcome. Read more about this topic in our guide Nominal flow rate.

The terms volume flow or flow rate also refer to the amount of gas moved per one unit of time. 

How does a fan move air?

The fan, no matter if it is mounted to the ceiling or a small shaft, creates a flow by rotating the blades. This flow sucks in air mass and circulates it. The more physical pressure a fan builds up, the better it can overcome long and difficult air ducts. Centrifugal fans are equipped with blades that resemble a paddle-wheel. Unlike axial models, they do not transport the air through their central axis but instead lead it through a pre-installed air duct in a 45° angle. The built-up air pressure is intensified by this angle so that centrifugal fans are predominantly used in industrial or professional environments, such as canteen kitchens or to supply an entire building.

Housing complexes with elongated shapes or nooks and crannies profit from the use of several smaller fans instead on one extra large device. More information about the perfect size of the fan is available in our guide section.