Duct Friction Loss Calculator: IP & SI Units

Use the duct friction loss calculator below to calculate the friction loss of rectangular ducts using airflow and duct size. Supports IP units and SI units with friction loss in in.wg/100 ft or Pa/m.

Duct Width

Duct Height

Airflow

CFM

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

Friction Loss	General Interpretation
Below 0.05 in.wg/100 ft / 0.4 Pa/m	Low friction loss
0.05–0.10 in.wg/100 ft / 0.4–0.8 Pa/m	Common design range
0.10–0.15 in.wg/100 ft / 0.8–1.2 Pa/m	Higher friction loss
Above 0.15 in.wg/100 ft / above 1.2 Pa/m	Consider larger duct or review design

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:

$D_{e} = \frac{1.3 (a b)^{0.625}}{(a + b)^{0.25}}$

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: $A = π {(\frac{D_{e}}{2})}^{2}$

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

$Δ P = f \cdot \frac{L}{D_{e}} \cdot \frac{ρ V^{2}}{2}$

Where:

$A$ = equivalent round duct area
$V$ = air velocity
$Q$ = airflow
$f$ = friction factor
$L$ = duct length
$ρ$ = air density
$Δ P$ = 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.

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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

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FAQ

What is duct friction loss?

Duct friction loss is the pressure loss caused by air rubbing against the duct surface as it flows through the duct.

What units does this calculator support?

The calculator supports IP and SI units. In IP mode, duct size is in inches, airflow is in CFM, and friction loss is in in.wg/100 ft. In SI mode, duct size is in millimeters, airflow is in L/s, and friction loss is in Pa/m.

Is lower friction loss always better?

Not always. Lower friction loss usually means larger ducts, which may reduce pressure loss and noise but require more space and material cost.

What is a typical duct friction rate?

A common design range is around 0.05–0.10 in.wg/100 ft, or approximately 0.4–0.8 Pa/m, but the best value depends on the project.

Does this include fittings and elbows?

No. This calculator estimates straight duct friction loss only. Fittings, elbows, transitions, dampers, grilles, diffusers, and filters add additional pressure loss.

Can I use this for final duct design?

Use it as a design reference. Final duct design should consider total external static pressure, fitting losses, fan selection, acoustic limits, space constraints, and project requirements.

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