What Is the Duct CFM & Airflow Velocity Calculator?
CFM (Cubic Feet per Minute) measures the volume of air circulating through an HVAC duct network every minute.
FPM (Feet per Minute) measures how fast that air is physically moving.
How Does the Duct CFM & Airflow Velocity Calculator Work?
The calculator computes duct internal cross-sectional area in square inches and divides by 144 to obtain square feet.
Multiplying square feet by FPM yields CFM.
Duct CFM & Airflow Velocity Calculator Formula & Variables
The core mathematical equation utilized by this calculator is expressed as:
CFM = Velocity (FPM) × Duct Area (sq ft)
Air volume equals linear flow velocity multiplied by the cross-sectional area of the conduit in square feet.
How to Use the Duct CFM & Airflow Velocity Calculator
- Select round or rectangular duct shape.
- Enter physical dimensions in inches.
- Specify airflow velocity in FPM.
Step-by-Step Example Calculation
12-inch Round Duct at 800 FPM Velocity
Input Values:
shape:round
dimension1Inches:12
dimension2Inches:0
velocityFpm:800
Understanding Your Result
Shows volumetric flow rate in CFM and metric cubic meters per hour.
Factors That Affect the Result
- Friction losses from flexible duct sag, elbows, dampers, and branch takeoffs.
When Should You Use This Calculator?
- Balancing air registers, sizing return grilles, and planning duct layout retrofits.
Assumptions & Limitations
- Assumes clean, unobstructed ductwork with uniform velocity profile across the cross-section.
Frequently Asked Questions
Calculation Accuracy & Reference Note
Standard kinematic fluid continuity equation Q = v × A.