How to Calculate Duct Friction Loss
Friction loss is the pressure the air gives up rubbing along the duct wall. This guide walks the Darcy-Weisbach method end to end — velocity, velocity pressure, friction factor, friction rate, then total loss on a run — with a worked commercial trunk and the assumptions behind every number.
Last updated September 2026
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The equation
All duct friction loss comes from one relationship. Everything else on this page is a way of getting its terms.
Δp is the pressure loss over the run, f the dimensionless friction factor, L the duct length, D the diameter (or equivalent diameter for rectangular and flat oval), and Pv the velocity pressure of the air in the duct.
Step 1 — Get the velocity
Velocity is airflow divided by the cross-sectional area. Area in square feet, airflow in CFM, velocity in feet per minute.
For a 22 in round duct carrying 4,000 CFM: the area is 2.64 sq ft, so the velocity is 1,515 FPM.
Step 2 — Get the friction factor
The friction factor depends on how rough the duct wall is relative to its diameter, and on the Reynolds number of the flow. The Altshul-Tsal approximation is the one used throughout this site because it is explicit — no iteration required.
In the 22 in example the Reynolds number is about 283,755 and the friction factor works out to 0.0160.
Step 3 — Convert to a friction rate
Designers rarely quote a raw pressure loss; they quote a friction rate in inches of water gauge per 100 feet of duct, so it can be applied to any run length.
The 22 in trunk gives 0.125 in. w.g. per 100 ft with a velocity pressure of 0.143 in. w.g.
Step 4 — Total the run
Multiply the friction rate by the straight duct length, then add every fitting on the path. Straight duct is often the smaller half of the total.
| Component | Quantity | Loss (in. w.g.) |
|---|---|---|
| Straight 22 in duct | 120 ft | 0.150 |
| 90° smooth elbow, r/D 1.5 (C 0.15) | 3 | 0.064 |
| Branch tee, flow into branch (C 1.0) | 1 | 0.143 |
| Volume damper, open (C 0.2) | 2 | 0.057 |
| Run total | — | 0.414 |
22 in spiral galvanized duct, 4,000 CFM, 70 °F standard air at sea level.
Run the same numbers with your own material, temperature and elevation in the fitting loss calculator, or size the duct first with the duct sizer.
Friction rate at 1,000 CFM by size
| Round size | Velocity | Spiral steel (in. w.g./100 ft) | Smooth PVC |
|---|---|---|---|
| 6 in | 5,093 fpm | 6.065 | 4.852 |
| 8 in | 2,865 fpm | 1.413 | 1.130 |
| 10 in | 1,833 fpm | 0.463 | 0.370 |
| 12 in | 1,273 fpm | 0.188 | 0.150 |
| 14 in | 935 fpm | 0.088 | 0.070 |
| 16 in | 716 fpm | 0.046 | 0.037 |
| 20 in | 458 fpm | 0.016 | 0.012 |
| 24 in | 318 fpm | 0.006 | 0.005 |
1,000 CFM, 70 °F standard air. Each step up in diameter roughly halves the friction rate.
Method and assumptions
- Method
- Darcy-Weisbach with the Altshul-Tsal explicit friction factor, ASHRAE Fundamentals Chapter 21 basis.
- Air properties
- Density from temperature and elevation via the standard atmosphere; viscosity from the Sutherland relation. Defaults are 70 °F at sea level.
- Roughness
- Absolute roughness ε by material, 0.01 mm (PVC) to 6 mm (flex at 70% extension).
- Rectangular duct
- Converted to a Huebscher equivalent diameter before the friction calculation.
- Units and rounding
- Imperial in and out, SI internally. Friction rates shown to three decimals; diameters are exact, not rounded to stock sizes.
Limitations of this method
What the calculation on this page does not account for. Read these before using a number on a drawing or a submittal.
- The friction rate applies to clean, straight, well-sealed duct. Dented, dirty, poorly joined or sagging duct loses more than the calculation shows.
- Fitting losses use published coefficients for idealised geometry. A field-fabricated elbow, a take-off cut short, or an elbow within a few diameters of another fitting will exceed them.
- Interaction effects are ignored. Fittings close together disturb each other's flow and their combined loss is higher than the sum of the individual coefficients.
- Leakage is not included. A duct system at its allowable leakage class still moves less air at the terminal than at the fan.
- This is a single-path calculation. It does not size a system, identify the critical path or check acoustics, and it is not a substitute for an engineered design stamped by a qualified professional.
Frequently asked questions
How do you calculate duct friction loss?
Multiply the friction rate of the duct, in inches of water gauge per 100 feet, by the length of the run in hundreds of feet, then add the loss of every fitting on that run. The friction rate itself comes from the Darcy-Weisbach equation using the duct diameter, air velocity, air density and the roughness of the duct material.
What is the Darcy-Weisbach equation for ductwork?
Pressure loss equals f times L divided by D, times the velocity pressure. f is the friction factor, L is the duct length, D is the hydraulic or equivalent diameter and the velocity pressure is the dynamic pressure of the moving air.
Does duct material change friction loss?
Yes. Absolute roughness ranges from about 0.01 mm for PVC and 0.09 mm for spiral galvanized steel up to 3 to 6 mm for flexible duct. A flexible duct run can lose two to three times as much pressure as the same diameter in spiral steel.