What is a normal duct velocity for residential HVAC?

ASHRAE recommends 600–900 FPM for main supply ducts in residences, and 400–700 FPM for branch ducts and return air. Velocities above 1,000 FPM in residential systems often cause audible noise.

What is the hydraulic diameter used for rectangular ducts?

The hydraulic diameter (Dh) equals 4 × cross-sectional area divided by the wetted perimeter: Dh = 4WH / (2(W+H)) = 2WH/(W+H). This converts rectangular geometry into an equivalent round diameter for friction calculations.

What standard air density is assumed?

Calculations assume standard air at 70°F and sea level: density 0.075 lb/ft³, kinematic viscosity 1.6 × 10⁻⁴ ft²/s. For high-altitude or high-temperature applications, results should be corrected accordingly.

How is friction loss calculated?

The tool uses the Darcy-Weisbach equation: ΔP = f × (L/Dh) × (ρV²/2). The Darcy friction factor f is found from the Colebrook-White equation assuming smooth ducts (ε = 0.0003 ft for sheet metal).

Does this include fittings (elbows, tees) pressure loss?

No. This calculator gives straight-duct friction loss only. Add fitting losses separately using equivalent-length or loss-coefficient (K-factor) methods.

What units does the calculator use?

Inputs are in CFM (airflow), inches (duct dimensions). Outputs are FPM (velocity) and inches of water gauge per 100 ft (friction loss). Area is shown in square inches and square feet.

Duct Velocity Calculator

Enter airflow (CFM) and duct size to instantly calculate air velocity (FPM) and friction loss (in. w.g. per 100 ft) for round or rectangular HVAC ducts.

Duct Parameters

Duct Shape

Airflow (CFM)

Duct Diameter (inches)

ASHRAE Velocity Guide

Main supply (commercial)1000–1500 FPM

Main supply (residential)600–900 FPM

Branch ducts400–700 FPM

Return air ducts300–600 FPM

Enter duct parameters and click Calculate

Summary

The Duct Velocity Calculator computes air velocity in feet per minute (FPM) and friction pressure loss (inches of water gauge per 100 ft) for both round and rectangular HVAC ducts. It uses the Darcy-Weisbach equation with the Colebrook-White friction factor for accurate results. Ideal for HVAC engineers, mechanical contractors, and energy auditors sizing duct systems.

How it works

Select duct shape: round (enter diameter) or rectangular (enter width and height).
Enter the airflow rate in CFM (cubic feet per minute).
The calculator converts rectangular ducts to hydraulic diameter for unified calculations.
Air velocity is derived from CFM divided by cross-sectional area.
Friction loss per 100 ft is calculated using the Darcy-Weisbach equation with air density at standard conditions (0.075 lb/ft³).
Results are displayed with ASHRAE recommended velocity range guidance.

Use cases

Size supply and return air ducts in residential and commercial HVAC systems.
Verify that duct velocities stay within ASHRAE recommended limits to minimize noise.
Estimate pressure drop across duct runs to select the correct fan/blower.
Cross-check contractor duct sizing proposals against design specifications.
Teach HVAC fundamentals in classrooms or training programs.
Troubleshoot high-velocity noise issues in existing duct systems.
Compare round vs. rectangular duct options for the same airflow.
Quick field calculations for mechanical engineers on job sites.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu