Duct Velocity Calculator (FPM)
Calculate air velocity based on CFM and duct dimensions for HVAC design.
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Round
Calculated Air Velocity: 0 FPM
How to Calculate Duct Velocity
In HVAC systems, duct velocity refers to the speed at which air travels through the ductwork. It is measured in Feet Per Minute (FPM). Calculating velocity is critical to ensure the system operates quietly, efficiently, and provides proper air distribution.
The core formula for air velocity is:
V = Q / A
- V: Velocity in feet per minute (FPM)
- Q: Airflow volume in cubic feet per minute (CFM)
- A: Cross-sectional area of the duct in square feet (sq. ft.)
Step-by-Step Calculation Example
Suppose you have a 12-inch round duct moving 800 CFM of air.
- Calculate Area (A): Area of a circle = π × r². Radius is 6 inches (0.5 feet). Area = 3.14159 × (0.5)² = 0.785 sq. ft.
- Calculate Velocity (V): V = 800 CFM / 0.785 sq. ft. = 1,019 FPM.
Recommended Duct Velocities
Choosing the right velocity is a balancing act. High velocities lead to noise and high pressure drop, while extremely low velocities may lead to temperature loss in the duct.
| Application | Recommended Velocity (FPM) |
|---|---|
| Residential Main Trunks | 700 – 900 FPM |
| Residential Branch Ducts | 600 FPM |
| Commercial Main Trunks | 1,000 – 1,500 FPM |
| Industrial/Heavy Commercial | 1,500 – 2,500 FPM |
Why Air Velocity Matters
If the velocity is too high (typically above 1,200 FPM in residential settings), you will hear "wind noise" or whistling at the registers. High velocity also increases static pressure, which forces the blower motor to work harder, shortening its lifespan and increasing energy bills. Conversely, if velocity is too low, air may not "throw" far enough into the room to achieve proper mixing.