Duct Velocity Calculator Formula
Understand the math behind the duct velocity calculator. Each variable explained with a worked example.
Formulas Used
Air Velocity
velocity = duct_area > 0 ? cfm / duct_area : 0Duct Area
duct_area_sqin = pi * pow(duct_diameter / 2, 2)Noise Risk
noise_level = duct_area > 0 && cfm / duct_area > 900 ? 1 : 0Variables
| Variable | Description | Default |
|---|---|---|
cfm | Airflow(CFM) | 300 |
duct_diameter | Duct Diameter(in) | 8 |
duct_area | Derived value= pi * pow(duct_diameter / 24, 2) | calculated |
How It Works
Duct Air Velocity
V = CFM / A
Where A is the cross-sectional area of the duct in square feet. Convert the diameter from inches to feet before calculating the area. Residential duct velocity should stay below 700-900 ft/min to avoid noise.
Worked Example
300 CFM through an 8-inch round duct.
cfm = 300duct_diameter = 8
- 01Area = pi x (8/24) squared = 0.349 sq ft
- 02Velocity = 300 / 0.349 = 860 ft/min
- 03This is borderline for noise; consider a 10-inch duct.
Frequently Asked Questions
What velocity causes noise?
Below 600 ft/min is quiet. 600-900 ft/min is acceptable. Above 900 ft/min produces noticeable noise. Supply registers should be under 500 ft/min.
How does velocity affect static pressure?
Higher velocity increases friction and pressure drop per foot of duct. This means the blower must work harder, reducing efficiency and increasing operating cost.
Ready to run the numbers?
Open Duct Velocity Calculator