Air Exchange Rate Calculator
About Air Exchange Rate Calculation
This calculator determines the required air exchange rate for room ventilation. It is used for preliminary selection of supply or exhaust system capacity and for checking air exchange based on air change rate or occupant count.
The result is provided in m³/h and additionally converted to L/s to make it easier to compare with equipment datasheets and design values.
Guidelines and recommendations
Calculation by air change rate
Method idea. First, the room volume is calculated. Then it is multiplied by the selected air change rate. The air change rate shows how many times the air in the room should be renewed in 1 hour.
Input data. Room area S in m2. Room height H in m. Air change rate n in 1/h.
V = S · H
L = n · V = n · S · H
Units. If the height is entered in centimeters, it is converted as H(m) = H(cm) / 100. The resulting airflow L is obtained in m3/h.
Practical benchmark. Commonly used values are n = 0.5…1.0 1/h for basic ventilation in residential rooms and n = 2…6 1/h for rooms with increased moisture or odors. Specific values are chosen based on the room function and the required indoor air quality category.
Calculation by number of people
Method idea. The airflow rate is calculated as the number of people multiplied by the design airflow per person. This method is useful when occupancy is known and is the governing factor.
Input data. Number of people N. Airflow per person q in m3/h·person.
L = N · q
Adopted values. The calculator uses fixed benchmarks: q = 20 m3/h·person (low activity), q = 40 m3/h·person (moderate activity), q = 60 m3/h·person (high activity).
Selecting the final value when different approaches are available
Selection principle. In practice, airflow is often calculated in several ways and the higher value is taken as the required system capacity to provide a margin for indoor air quality.
How to compare results. If the air change rate method gives a higher value than the people-based method, the room volume and renewal frequency are typically governing. If the people-based method is higher, occupancy density and occupant emissions are governing.
Unit conversion of the result
Converting m³/h to L/s. To compare with some references and ventilation equipment, the airflow is converted as follows:
L(L/s) = L(m³/h) / 3.6
Rounding. The value in L/s is rounded to 0.01. The value in m³/h may be used as the design value without additional rounding when selecting equipment.
Assumptions used in the calculation
Uniform mixing. It is assumed that the air in the room is well mixed and that indoor air quality can be represented by an average supply or exhaust airflow rate.
Steady-state operation. The calculation corresponds to a constant load and does not account for short-term peaks, window airing, or daily occupancy schedules.
No allowance for leakage and imbalance. Infiltration, transfer through gaps, and supply-exhaust imbalance are not added as separate factors. If they are significant, the airflow rate is typically increased by a design margin.
European reference standards for choosing parameters
EN 16798-1. “Energy performance of buildings. Ventilation for buildings. Input parameters for the indoor environment” is used to select indoor air quality categories and related ventilation requirements.
ISO 17772-1. “Energy performance of buildings. Indoor environmental input parameters” is used as a base reference for defining target indoor environment parameters and approaches to ensuring indoor air quality.
EN 16798-3. “Ventilation for non-residential buildings. Performance requirements for ventilation and room-conditioning systems” supports interpretation of airflow requirements and design solutions for ventilation systems.
FAQs
Why can airflow be calculated by air change rate and by number of people, and the results differ?
The methods reflect different governing factors. The air change rate method is tied to room volume and renewal frequency, while the people-based method is tied to occupant-related emissions. In design practice, both results are often compared and the higher airflow is selected.
What air change rate is considered “normal”?
There is no single value for all rooms because the requirement depends on room function and the indoor air quality category. Common benchmarks are around 0.5…1.0 1/h for basic ventilation and higher values for rooms with increased moisture or odors. Specific targets are selected based on the room function and approaches in EN 16798-1 and ISO 17772-1.
What does the result in L/s mean, and why is it needed?
Liters per second are often used in references and in descriptions of ventilation components. The conversion is performed using L(L/s) = L(m³/h) / 3.6. It is the same airflow rate expressed in a different unit.
Can I select a fan directly from this calculation?
The airflow rate defines the required capacity by volume, but fan selection usually also requires pressure-loss estimation in the duct network and the equipment performance curve. For a quick preliminary choice, the airflow is taken with a margin and then refined using an aerodynamic calculation and the fan operating point.
Why does the calculation not include infiltration and window airing?
This is a simplified ventilation calculation intended to determine the required airflow rate according to the selected criterion. Infiltration and window airing depend strongly on the building, wind, and operating conditions. If they are important, airflow is typically adjusted by a design margin or assessed with a separate calculation.