Duct Equal Friction & Velocity Calculator

Size circular and rectangular air distribution ducts using the standard ASHRAE equal friction method with velocity, aspect ratio, and noise guidelines.

Airflow & Design Friction Rate

Airflow Rate Q (CFM):

Friction Rate (in. w.g. / 100 ft):

Duct Run Length (ft):

Duct Material / Roughness:

Rectangular Aspect Constraint

Constraint Mode:

Fixed Height (in) / Aspect Ratio:

Duct Sizing Dimensions

Equivalent Round Duct Diameter

-- inches

Rectangular: --" W × --" H

Air Velocity (V)

-- FPM

Total Duct Friction Loss

-- in. w.g.

across total run length

Rectangular Aspect Ratio: -- : 1

Velocity Pressure (Pv): -- in. w.g.

Hydraulic Diameter (Dh): -- inches

Equal friction sizing ensures uniform pressure drop per 100 ft across all main trunk and branch lines.

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Frequently Asked Questions

What is the equal friction method of duct design?

The equal friction method sizes ductwork so that the friction loss per unit length (typically 0.08 to 0.15 inches water gauge per 100 feet) remains completely constant throughout the entire system. As airflow decreases past downstream takeoffs, the duct diameter automatically shrinks.

Why should duct aspect ratio be kept below 4:1?

A square duct (1:1 aspect ratio) encloses maximum cross-sectional area with minimum perimeter sheet metal. Wide flat ducts with aspect ratios above 4:1 dramatically increase sheet metal cost, surface heat transfer loss, and structural oil-canning vibration.

What are typical maximum air velocity limits for HVAC noise criteria?

ASHRAE recommends maximum duct velocities of 700 - 900 FPM for branch runouts to diffusers, 1,000 - 1,400 FPM for commercial main supply trunks, and up to 2,000 - 2,500 FPM for industrial plants and mechanical equipment rooms.