Duct Resistance Calculator: Estimate Pressure Loss

A duct resistance calculator estimates the pressure lost as air moves through a straight duct, based on airflow, duct size, length, and air properties. Enter those values to get an approximate pressure drop; use it for preliminary sizing, then account separately for fittings and other system components.

Formula

This calculator uses the Darcy–Weisbach equation for friction loss in a straight, circular duct:

Δp = f × (L / D) × (ρ × v² / 2), with v = Q / A = 4Q / (πD²).

• Δp is pressure loss, in pascals (Pa).
• f is the Darcy friction factor, dimensionless.
• L is straight duct length, in metres (m).
• D is the duct’s inside diameter, in metres (m).
• ρ is air density, in kilograms per cubic metre (kg/m³).
• v is average air velocity, in metres per second (m/s).
• Q is volumetric airflow, in cubic metres per second (m³/s), and A is the duct’s cross-sectional area, in square metres (m²).

The friction factor in this model is the Darcy factor, not the Fanning factor; the Darcy value is four times the Fanning value for the same flow. The calculator accepts a friction factor rather than deriving one from duct roughness and Reynolds number. Its answer is therefore only as reliable as that input.

Worked Example

Using the defaults—airflow 0.5 m³/s, inside diameter 0.3 m, length 10 m, density 1.2 kg/m³, and Darcy friction factor 0.02:

1. Area = π × 0.3² / 4 ≈ 0.07069 m².
2. Velocity = 0.5 / 0.07069 ≈ 7.07 m/s.
3. Pressure loss = 0.02 × (10 / 0.3) × (1.2 × 7.07² / 2) ≈ 200 Pa.
4. Loss per length = 200 / 10 = 20.00 Pa/m.

The calculator rounds pressure loss to 200.00 Pa, mean velocity to 7.07 m/s, and pressure loss per length to 20.00 Pa/m. The unrounded pressure calculation is approximately 200.0 Pa.

What Duct Resistance Means

Air loses pressure as it rubs against duct walls and moves through restrictions. A fan must provide enough pressure to overcome the resistance of the complete air path while delivering the required airflow. Duct pressure loss is also called friction loss or frictional pressure drop.

The result here covers friction along one straight, round duct section only. A complete ventilation system can include elbows, tees, transitions, dampers, filters, grilles, coils, and terminal devices. These create additional losses, commonly calculated with fitting loss coefficients or manufacturer data and added to the straight-duct losses. The total system pressure requirement is not the same as this calculator’s result.

How to Use the Result

Use the pressure loss to compare candidate duct sizes or estimate how much pressure a straight section consumes. The pressure-loss-per-length output can help compare runs of different lengths under the same assumptions. At a fixed airflow, reducing duct diameter raises velocity sharply, and friction loss can rise substantially; increasing diameter often reduces loss but may require more space and material.

For US customary units, convert inputs before using this version: airflow in cubic feet per minute to m³/s by multiplying by 0.000471947; diameter and length in feet to metres by multiplying by 0.3048; and air density in lb/ft³ to kg/m³ by multiplying by 16.0185. The outputs are in pascals and metres per second. One inch of water column is approximately 249.1 Pa, and one metre per second is approximately 196.85 feet per minute. Keep conversions consistent rather than entering a value in one unit while labeling it as another.

Choosing a Friction Factor

The Darcy friction factor depends on flow regime, internal surface roughness, and Reynolds number. It is not a universal constant for all ducts. Smooth, straight ducts may have lower friction than rough or internally lined ducts, while low-flow conditions can behave differently from turbulent flow. For a detailed design, determine the factor from an accepted duct-friction method, chart, or engineering calculation using the actual material and operating conditions.

Many practical HVAC calculations use duct-friction charts or tables that relate airflow, duct size, and pressure loss for a stated duct type and assumed air conditions. Such methods may be more convenient than separately estimating a friction factor. Confirm what the chart includes and whether it represents straight duct only.

Common Errors and Limitations

• Using the wrong diameter: Use the clear inside diameter, not the outside diameter. For rectangular ducts, an equivalent diameter or a rectangular-duct method is needed; simply substituting one side is not valid.
• Mixing friction-factor conventions: Enter a Darcy factor. A Fanning factor used directly would understate the calculated loss by a factor of four.
• Counting only straight runs: Add fitting and equipment losses separately. A route with several elbows can have materially more resistance than a straight-run estimate suggests.
• Assuming air conditions are fixed: Density varies with temperature, pressure, and altitude. The default density is a convenient approximation, not a value suitable for every site.
• Treating an estimate as a fan selection: Fan performance depends on the full system curve and required airflow. This calculation alone does not establish fan size, sound level, energy use, or code compliance.

The equation assumes steady flow and uses average velocity. Real installations may have nonuniform flow, leakage, flexible-duct compression, rough surfaces, or installation conditions that change resistance. Rounding is usually less important than choosing sound input values and including every relevant component.

Frequently Asked Questions

What does a duct resistance calculator calculate?
It estimates frictional pressure loss in a stated length of straight duct from airflow, duct dimensions, air density, and a friction factor. It does not automatically include fittings or equipment.

Is duct resistance the same as pressure drop?
In this context, duct resistance describes how strongly the duct opposes airflow, while pressure drop is the resulting pressure difference at the specified airflow. Resistance is not a single fixed pressure value independent of flow.

Can I use this calculator for rectangular ducts?
Not by entering one side as the diameter. Rectangular ducts need an appropriate equivalent-diameter approach or a method that directly accounts for width and height, as well as the duct’s construction.

What friction factor should I enter?
Use a Darcy friction factor appropriate to the duct material, roughness, and flow regime. If you do not know it, consult a suitable friction chart or duct-design method rather than assuming one value fits every installation.

Does the result include elbows and filters?
No. It covers straight-duct wall friction only. Estimate or obtain pressure losses for elbows, filters, dampers, coils, grilles, and other components, then combine them for a system-level estimate.

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in q | Airflow | m3/s | 0.5 | 0
in d | Duct inside diameter | m | 0.3 | 0
in l | Straight duct length | m | 10 | 0
in rho | Air density | kg/m3 | 1.2 | 0
in f | Darcy friction factor | - | 0.02 | 0
out dp | Straight-duct pressure loss | Pa | f * (l / d) * (rho * (4 * q / (pi * d ^ 2)) ^ 2 / 2) | 2
out v | Mean air velocity | m/s | 4 * q / (pi * d ^ 2) | 2
out iw | Pressure loss per length | Pa/m | dp / l | 2