Duct Velocity Calculator (CFM to FPM)
Calculate the air velocity in your HVAC ductwork based on airflow (CFM) and duct dimensions. This tool supports both rectangular and round duct profiles.
Air Velocity:
Understanding Duct Velocity
In HVAC (Heating, Ventilation, and Air Conditioning) design, duct velocity refers to how fast air moves through the ductwork, measured in Feet Per Minute (FPM). It is a critical factor in determining the efficiency, noise levels, and pressure drop of a mechanical system.
The Duct Velocity Formula
The fundamental equation used by this calculator is:
V = Q / A
- V: Velocity in feet per minute (FPM)
- Q: Airflow rate in cubic feet per minute (CFM)
- A: Cross-sectional area of the duct in square feet (sq ft)
Recommended Duct Velocity FPM Limits
Designing a system with too high a velocity can lead to excessive noise (whistling or rushing sounds) and increased energy consumption due to higher static pressure. Conversely, too low a velocity can lead to poor air distribution and debris buildup.
| Application | Recommended Velocity (FPM) |
|---|---|
| Residential Main Ducts | 700 – 900 FPM |
| Residential Branch Ducts | 600 FPM |
| Commercial Main Ducts | 1,000 – 1,500 FPM |
| Industrial Main Ducts | 1,500 – 2,500 FPM |
| Kitchen Exhaust Ducts | 1,500 – 2,000 FPM (to keep grease suspended) |
Calculation Example
Suppose you have a furnace pushing 1,200 CFM of air into a rectangular main trunk that measures 20 inches by 10 inches.
- Calculate Area in Square Inches: 20″ × 10″ = 200 sq. in.
- Convert Area to Square Feet: 200 / 144 = 1.389 sq. ft.
- Calculate Velocity: 1,200 CFM / 1.389 sq. ft. = 864 FPM.
This result (864 FPM) falls within the ideal range for a residential main duct.