What is friction loss in a duct?

Friction loss (pressure drop) is the reduction in static pressure caused by air rubbing against the duct walls as it travels along the duct. It depends on duct velocity, length, diameter, and the roughness of the duct surface.

What equation does this calculator use?

It uses the Darcy-Weisbach equation: ΔP = f × (L/Dh) × (ρV²/2). The Darcy friction factor f is solved from the Colebrook-White equation iteratively for turbulent flow, or as 64/Re for laminar flow (Re < 2,300).

What roughness value should I use?

Galvanized sheet metal ducts use ε = 0.0003 ft (0.00009 m). Flexible duct is rougher at about 0.003 ft. Fibrous ductboard is roughest at around 0.015 ft. The calculator defaults to sheet metal (0.0003 ft).

Does this include fitting losses?

No. This tool calculates straight-duct friction loss only. For elbows, tees, transitions, and other fittings, add losses using the equivalent-length method or loss-coefficient (K-factor) method on top of the straight-duct result.

What air properties are assumed?

Standard air at 70°F (21°C) and sea level: density 0.075 lb/ft³ (1.2 kg/m³), kinematic viscosity 1.6 × 10⁻⁴ ft²/s. For high-altitude or high-temperature systems, density correction is needed.

What is hydraulic diameter for rectangular ducts?

Hydraulic diameter Dh = 2WH/(W+H), where W and H are duct width and height. It converts rectangular geometry to an equivalent circular diameter for friction calculations. This is the standard approach per ASHRAE Fundamentals.

Duct Friction Loss Calculator

Name: Duct Friction Loss Calculator
Availability: InStock
Author: Nham Vu

Enter duct shape, airflow, dimensions, and length to calculate total friction pressure loss in inches of water gauge using the Darcy-Weisbach equation.

Duct Parameters

Duct Shape

Airflow (CFM)

Duct Diameter (inches)

Duct Length (feet)

Duct Material / Roughness

Enter duct parameters and click Calculate

Summary

Enter duct shape, airflow, dimensions, and length to calculate total friction pressure loss in inches of water gauge using the Darcy-Weisbach equation.

How it works

Select duct shape — round (enter diameter) or rectangular (enter width and height).
Enter the airflow rate in CFM and the duct run length in feet.
Optionally adjust the duct material roughness (default: 0.0003 ft for galvanized sheet metal).
The calculator converts rectangular ducts to an equivalent hydraulic diameter.
Air velocity is computed from CFM divided by cross-sectional area.
The Reynolds number and Colebrook-White friction factor are solved iteratively.
Total friction loss is calculated via Darcy-Weisbach: ΔP = f × (L/Dh) × (ρV²/2).
Results are displayed in in. w.g. and Pascals alongside key intermediate values.

Use cases

Size the fan or blower needed to overcome friction losses in a duct system.
Verify that duct runs meet maximum pressure-drop-per-100-ft design criteria.
Compare galvanized steel, flexible, and fibrous duct materials by roughness.
Balance multi-branch HVAC systems by calculating pressure loss in each branch.
Validate contractor duct design against ASHRAE and SMACNA guidelines.
Teach Darcy-Weisbach and Colebrook-White methods in HVAC engineering courses.
Quickly estimate pressure loss in field conditions without software.
Cross-check modeled results from full HVAC design software.

Frequently Asked Questions

Last updated: 2026-06-10 · Reviewed by Nham Vu