Duct Friction Loss Calculator

Calculate duct friction loss for rectangular ducts using airflow and duct size. Supports IP units and SI units with friction loss in in.wg/100 ft or Pa/m.

Note: This calculator is intended for quick HVAC design reference. Final duct sizing should also consider velocity, noise, available fan static pressure, fittings, duct layout, and project design requirements.

What the Results Mean

• Friction Loss
  Friction loss shows how much pressure the air loses as it moves through a duct. Higher friction loss means the fan must work harder to deliver the required airflow.
• Equivalent Diameter
  Equivalent diameter converts a rectangular duct into an equivalent round duct size for friction calculation purposes.
• Velocity
  Velocity shows how fast air is moving through the duct. High velocity can increase friction loss and noise.
• Friction Factor
  Friction factor is used in the pressure loss calculation and depends on duct size, airflow condition, and surface roughness.

Typical Duct Friction Loss Range

These are general reference ranges. Actual acceptable values depend on building type, noise criteria, available space, fan selection, and design method.

Example Calculation

Given:

• Duct size: 12 in × 12 in
• Airflow: 300 CFM

Result:

• Friction loss: about 0.014 in.wg/100 ft
• Equivalent diameter: about 13.1 in
• Velocity: about 320 fpm

This is a low-friction condition because the airflow is relatively small for the duct size.

Formula Used

For rectangular duct, the equivalent diameter is calculated as:

$$D_e=\frac{1.3(ab{)}^{0.625}}{(a+b{)}^{0.25}}$$

Where:

• $D_e$ = equivalent duct diameter
• $a$ = duct width
• $b$ = duct height

The friction loss is then calculated using airflow, equivalent diameter, air velocity, air density, duct roughness, and friction factor.

Simplified calculation flow:$$A=\pi {\left(\frac{D_e}{2}\right)}^2$$

$$V=\frac{Q}{A}$$

$$\mathrm{\Delta }P=f\cdot \frac{L}{D_e}\cdot \frac{\rho V^2}{2}$$

Where:

• $A$ = equivalent round duct area
• $V$ = air velocity
• $Q$ = airflow
• $f$ = friction factor
• $L$ = duct length
• $\rho$ = air density
• $\mathrm{\Delta }P$ = pressure loss

Note: The calculator uses the equivalent round duct method for rectangular ducts and converts SI inputs into IP units internally before converting the final friction loss back to Pa/m.

How to Use This Calculator

Use the calculator when you want to:

• Estimate duct pressure loss
• Compare different duct sizes
• Check whether duct velocity is reasonable
• Support early duct sizing decisions
• Review duct design assumptions
• Understand how airflow and duct size affect friction loss

Important Design Notes

Duct friction loss is only one part of duct design. A complete duct design should also consider:

• Duct fittings and elbows
• Transitions
• Branch takeoffs
• Dampers
• Diffusers and grilles
• Filters
• Flexible duct sections
• Available fan static pressure
• Noise and vibration
• Installation constraints

Related HVAC Calculators

• Duct Velocity Calculator
• Equivalent Duct Diameter Calculator
• Grille Size Calculator
• Chilled Water Pipe Sizing Calculator
• BTU to Ton / HP / kW Converter

Related Learning Guides

• HVAC Ductwork Design Guide
• Duct Velocity Pressure
• Static Pressure Explained
• Return Air Grille Sizing Guide
• Types of Ducts in HVAC
• Duct Elbows Explained

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