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)
- SP = Static pressure (inches water column)
- η = Fan efficiency (decimal)
- 6356 = unit conversion constant
- 1 BHP = 745.7 Watts
Assumptions & References
- Method 1 uses the standard ACH formula: CFM = (Volume × ACH) / 60 minutes per hour.
- Method 2 follows ASHRAE Standard 62.1-2022 Ventilation Rate Procedure for acceptable indoor air quality.
- Fan power calculation uses the Air Horsepower / BHP formula standard in HVAC engineering (AMCA, ASHRAE Fundamentals).
- Typical ACH values: Offices 6–10, Commercial kitchens 15–60, Bathrooms 6–10, Bedrooms 4–6, Hospital isolation rooms 12+.
- 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.
- Static pressure values for typical duct systems range from 0.1 to 1.0 in. WC; higher values indicate more restrictive ductwork.
- Centrifugal fan efficiencies typically range from 60–80%; axial fans 50–70%.
- This calculator does not account for duct leakage, altitude corrections, or system effect factors — apply appropriate safety factors (typically 10–25%) for final fan selection.