Duct Velocity Calculator Formula

Understand the math behind the duct velocity calculator. Each variable explained with a worked example.

Use the Duct Velocity Calculator

Formulas Used

Air Velocity

velocity = duct_area > 0 ? cfm / duct_area : 0

Duct Area

duct_area_sqin = pi * pow(duct_diameter / 2, 2)

Noise Risk

noise_level = duct_area > 0 && cfm / duct_area > 900 ? 1 : 0

Variables

VariableDescriptionDefault
cfmAirflow(CFM)300
duct_diameterDuct Diameter(in)8
duct_areaDerived 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
  1. 01Area = pi x (8/24) squared = 0.349 sq ft
  2. 02Velocity = 300 / 0.349 = 860 ft/min
  3. 03This is borderline for noise; consider a 10-inch duct.

Ready to run the numbers?

Open Duct Velocity Calculator