Airflow CFM Calculator — Fan and Ventilation Requirements
Calculate the required airflow in cubic feet per minute (CFM) for fans and ventilation systems using room volume, air changes per hour (ACH), occupancy load, or area-based methods per ASHRAE 62.1 standards.
Method 1: Room Volume & Air Changes per Hour (ACH)
Typical: Office 6–10, Kitchen 15–60, Bathroom 6–10, Bedroom 4–6
Method 2: ASHRAE 62.1 Occupancy-Based
ASHRAE 62.1: Office 5–20 CFM/person, Classroom 10–15, Conference 5–10
ASHRAE 62.1 default: 0.06 CFM/sq ft for offices
Fan Efficiency & Static Pressure
Typical duct systems: 0.1–1.0 in. WC
Typical centrifugal fans: 60–80%
Formulas Used
Method 1 — ACH Method:
CFM = (Room Volume [ft³] × ACH) / 60
Where: Room Volume = Length × Width × Height
Method 2 — ASHRAE 62.1 Ventilation Rate Procedure:
Vbz = (Rp × Pz) + (Ra × Az)
- Vbz = Breathing zone outdoor airflow (CFM)
- Rp = Outdoor air rate per person (CFM/person)
- Pz = Zone population (number of occupants)
- Ra = Outdoor air rate per unit area (CFM/ft²)
- Az = Zone floor area (ft²)
Fan Brake Horsepower:
BHP = (CFM × SP) / (6356 × η)
Assumptions & References
- Method 2 follows ASHRAE Standard 62.1-2022 Ventilation Rate Procedure for acceptable indoor air quality.
- ASHRAE 62.1 default Rp values: Office 5 CFM/person, Classroom 10 CFM/person, Conference room 5 CFM/person.
- ASHRAE 62.1 default Ra = 0.06 CFM/ft² for office spaces.
- When both methods are used, select the higher CFM value for design to satisfy both ventilation and air quality requirements.