Air Flow Calculator
An air flow calculator converts between CFM (cubic feet per minute), air velocity (FPM - feet per minute), and duct cross-sectional area. The fundamental relationship Q = A x V (flow equals area times velocity) is used for duct design, air balancing, fan selection, and HVAC commissioning. You can use this tool in two directions: enter duct dimensions and velocity to find CFM, or enter CFM and duct size to find the resulting velocity. This is essential when balancing air distribution systems, sizing diffusers and grilles, or troubleshooting rooms with insufficient airflow. Both round and rectangular duct options are provided.
Air flow formula
Round area (sq in) = pi x (D/2)^2
Rect area (sq in) = Width x Height
Area (sq ft) = Area (sq in) / 144
CFM = Area (sq ft) x Velocity (fpm)
Velocity Pressure (in wg) = (V / 4005)^2 [standard air at 70F]
Frequently asked questions
How do I calculate CFM from duct size and velocity?
CFM = Area (sq ft) x Velocity (fpm). For a round duct, Area = pi x (D/2)^2 / 144 sq ft where D is in inches. For a rectangular duct, Area = Width x Height / 144 sq ft. Multiply by the air velocity in feet per minute (fpm) to get cubic feet per minute (CFM).
What is a typical air velocity in supply ducts?
Supply main ducts: 700-900 fpm. Supply branch ducts: 400-700 fpm. Return air: 600-800 fpm. Residential supply outlets: 300-500 fpm. Higher velocities cause noise and increase fan energy; lower velocities require larger ducts. ASHRAE and ACCA provide velocity limits for each application.
What is the difference between CFM and fpm?
CFM (cubic feet per minute) is a measure of volume flow rate. FPM (feet per minute) is air velocity. CFM = FPM x Duct Area. An anemometer or pitot tube measures velocity (fpm), which is then multiplied by the duct area to calculate CFM. Balancing technicians use this method to measure and adjust air distribution.
How do I measure air flow in a duct?
Common field measurement methods include: (1) pitot tube traverse - measures velocity pressure at multiple points across the duct cross-section; (2) anemometer grid measurement; (3) flow hood placed over supply/return grilles for direct CFM reading. ASHRAE provides measurement procedures in ASHRAE Standard 111.
How many CFM per square foot of floor area is recommended?
ASHRAE 62.1 requires a minimum of 0.06 CFM per square foot of floor area plus 5 CFM per person for office buildings. Residential ventilation (ASHRAE 62.2) requires 1 CFM per 100 sq ft of floor area plus 7.5 CFM per person. Cooling systems typically deliver 0.5-1 CFM per sq ft of floor area.
Official sources
- ASHRAE: ASHRAE Standard 111 - Measurement, Testing, Adjusting, and Balancing of Building HVAC Systems.
- ASHRAE: ASHRAE Standard 62.1 - Ventilation for Acceptable Indoor Air Quality.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.