ASHRAE 62.1 Ventilation Calculator
Calculate the minimum breathing-zone outdoor airflow for a space using the ASHRAE 62.1 Ventilation Rate Procedure.
Unit system
Switching units converts your values — it never changes the requirement.
Minimum Breathing-Zone Outdoor Airflow
Select a space type
Calculated the airflow? Design the system.
Use MEPFlow to take your airflow calculations further with visual HVAC and duct-system design tools.
How the ASHRAE 62.1 Ventilation Calculator Works
Outdoor-air ventilation requirements generally contain two components: a people-related component that scales with the number of occupants, and an area-related component that scales with the floor area of the space. The Ventilation Rate Procedure adds the two together to produce the breathing-zone outdoor airflow.
Vbz = Rp × Pz + Ra × Az
- Vbz — breathing-zone outdoor airflow, CFM
- Rp — outdoor airflow rate per person, CFM/person
- Pz — design zone population, people
- Ra — outdoor airflow rate per unit floor area, CFM/ft²
- Az — zone floor area, ft²
Selecting a space type applies the corresponding people and area rates, so the only project data you need to enter is floor area and design occupancy.
What Is Breathing-Zone Outdoor Airflow?
Breathing-zone outdoor airflow, Vbz, is the quantity of outdoor air that must reach the occupied region of a zone — broadly, the volume where people actually breathe. It is the starting point of the Ventilation Rate Procedure and the value most often scheduled per room during design.
Why Do Floor Area and Occupancy Both Matter?
Occupants generate contaminants such as carbon dioxide, moisture and odours, so part of the requirement scales with population. The building itself also emits contaminants from finishes, furnishings and stored materials, so part of the requirement scales with floor area. A lightly occupied but large space can therefore still require meaningful outdoor airflow, and a small densely occupied space can require far more than its area alone would suggest.
What Is Zone Air Distribution Effectiveness?
Zone air distribution effectiveness, Ez, describes how effectively a particular supply-air arrangement delivers outdoor air to the breathing zone. Because Ez depends on the actual air-distribution configuration — overhead cooling, overhead heating, underfloor supply, displacement ventilation and so on — it is not a single fixed value, and this calculator does not assume one.
Vbz is therefore not necessarily identical to the final outdoor airflow that a given supply-air configuration must deliver. Determining zone outdoor airflow, Voz, and the system-level outdoor air intake requires additional calculations based on the selected system configuration.
Is This a Complete HVAC Ventilation Design?
No. This quick calculator determines the breathing-zone component from the configured space reference rates. A complete design may also require consideration of:
- air-distribution effectiveness
- system configuration
- multiple-zone calculations
- air density and elevation
- the adopted standard edition
- local codes
- the authority having jurisdiction
Frequently Asked Questions
What does CFM mean?
CFM is cubic feet per minute, the volumetric airflow rate used throughout imperial HVAC design. Ventilation requirements in this calculator are reported in CFM.
What is Rp?
Rp is the outdoor airflow rate required per person, in CFM per person. It scales with the number of people expected in the space.
What is Ra?
Ra is the outdoor airflow rate required per unit of floor area, in CFM per square foot. It accounts for emissions from the building itself, its finishes and its contents.
What occupancy should I enter?
Enter the design zone population for the space. Where a project has not established a population, designers commonly use the occupancy assumed by the applicable code or the default occupant density for that space type.
Can I use this for a multi-zone air-handling system?
This calculator returns the breathing-zone outdoor airflow for a single zone. Multiple-zone recirculating systems require additional system-level calculations to determine the outdoor air intake flow.
Does this calculator replace the ASHRAE standard?
No. It is an engineering design aid based on the configured reference rates. Always verify final requirements against the standard edition and local code adopted by the authority having jurisdiction.