Room Air Change Calculator
The airflow calculator below estimates the ventilation requirement for each room based on the selected Air Changes per Hour (ACH) value. ACH represents the number of times the total volume of air within a space is replaced in one hour and is a key parameter in ventilation design. Different room types require different ACH values depending on occupancy levels, heat gains, contaminants, moisture generation, and the intended use of the space. For example, offices and classrooms typically require moderate air change rates to maintain indoor air quality and occupant comfort, while kitchens, toilets, and healthcare spaces generally require significantly higher ventilation rates to remove odours, moisture, and airborne pollutants.
To calculate the required airflow, the room volume is multiplied by the chosen ACH rate and converted into either litres per second (L/s) or cubic metres per hour (m³/h). The resulting airflow can then be used for sizing ventilation systems, ductwork, grilles, and associated mechanical equipment. Recommended ACH values should be selected in accordance with relevant standards and guidance documents such as CIBSE Guide B2, CIBSE Guide A, Approved Document F, and project-specific requirements. Where occupancy or operational conditions vary, designers should apply engineering judgement and consider both minimum fresh air requirements and extract ventilation needs. Once the airflow has been obtained, the next step in the design process is to select the appropriate ventilation duct sizes, this can be done using the HVAC Duct Sizing calculator.
Recommended ACH values for different room types according to CIBSE Guide B2:
| Room Type | CIBSE Recommended ACH |
|---|---|
| Bedrooms & Living Rooms | 0.5 to 1.0 (or 3-5 infiltration max) |
| Kitchens and Bathrooms | 3.0 to 10.0 |
| General Offices | 4.0 to 8.0 |
| Meeting Rooms & Classrooms | 8.0 to 12.0 |
| Laboratories | 6.0 to 15.0 |
| Restaurants (Dining) | 8.0 to 10.0 |
| Restaurants (Kitchens) | 30.0 or more |