Duct Friction Rate: What It Means and How to Choose One
The friction rate is the single number that sets the size of every duct in an equal-friction system. This guide explains what it represents physically, what the common design ranges are and why, and how to derive a defensible value from the fan static pressure you actually have.
Last updated September 2026
Advertisement
What the number means
A friction rate of 0.10 in. w.g. per 100 ft means that for every 100 feet of straight duct the air travels, it loses a tenth of an inch of water gauge of static pressure to wall friction. It says nothing about fittings, and nothing on its own about whether the duct is the right size — it is a rate, and a rate only becomes a pressure when you multiply it by a length.
Picking a friction rate is a trade. A lower rate buys quieter, lower-energy systems in bigger, costlier duct. A higher rate buys compact duct that the fan has to work harder to serve.
What the same airflow looks like at each rate
| Friction rate | Round size for 2,000 CFM | Resulting velocity |
|---|---|---|
| 0.05 in. w.g./100 ft | 20.4 in | 882 fpm |
| 0.08 in. w.g./100 ft | 18.5 in | 1,068 fpm |
| 0.10 in. w.g./100 ft | 17.7 in | 1,169 fpm |
| 0.15 in. w.g./100 ft | 16.3 in | 1,378 fpm |
| 0.20 in. w.g./100 ft | 15.4 in | 1,548 fpm |
Spiral galvanized duct, 70 °F standard air. Exact diameters, before rounding to stock sizes.
Doubling the friction rate from 0.05 to 0.10 shrinks the duct by only a couple of inches but raises the velocity — and with it the noise and the fan energy — substantially. That asymmetry is why most of the argument in duct design happens over this one number.
Typical design ranges
| System type | Commonly designed at | Why |
|---|---|---|
| Residential supply | 0.08 – 0.10 | Matches the external static a residential air handler is rated for over short runs. |
| Residential return | 0.05 – 0.08 | Return paths are short and noise-sensitive at the grille. |
| Commercial low-noise | 0.06 – 0.10 | Offices, classrooms, healthcare — keeps trunk velocity under about 1,200 FPM. |
| Commercial general | 0.10 – 0.15 | Standard equal-friction practice for retail, warehouse and general office. |
| Flexible duct runouts | 0.05 – 0.08 | Flex is far rougher; designing it at metal-duct rates undersizes it badly. |
| Industrial / high velocity | 0.20+ | Space and first cost are prioritised over fan energy. |
Ranges in in. w.g. per 100 ft. These are commonly used values, not code requirements or universal targets.
Deriving the rate from the fan you have
The defensible way to choose a friction rate is to work backwards from the available static pressure rather than pick a habit number.
Take the fan's external static pressure rating, subtract everything that is not duct — coils, filters at their dirty pressure drop, louvres, dampers, terminals, attenuators — and divide what remains by the total effective length of the critical path, which is the straight duct plus the equivalent length of every fitting on it.
Worked: a unit rated at 1.00 in. w.g. external static, losing 0.45 in. w.g. to a dirty filter, coil and diffuser, leaves 0.55 in. w.g. for the duct. If the critical path has a total effective length of 420 ft, the friction rate is 0.55 ÷ 4.2 = 0.131 in. w.g. per 100 ft.
Size the ducts at that rate in the duct sizer, check the resulting velocities in the velocity calculator, and total the path in the fitting loss calculator.
Method and assumptions
- Method
- Darcy-Weisbach friction with the Altshul-Tsal friction factor; equal-friction sizing basis.
- Air properties
- 70 °F at sea level unless the calculator inputs say otherwise.
- Material
- Table values use spiral galvanized steel, ε = 0.09 mm.
- Rounding
- Diameters shown exact to one decimal; specify the next available stock size.
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.
- A friction rate is a starting assumption, not an outcome. It does not guarantee the fan can deliver design airflow until the critical path has been totalled.
- The ranges on this page are common industry practice, not code values, and not appropriate for every project. Project specifications, energy codes and acoustic criteria override them.
- Equal-friction sizing takes no account of how pressure divides between branches; short branches will over-deliver without balancing dampers.
- Applying a metal-duct friction rate to flexible duct, lined duct or duct board will undersize the run — those materials need their own roughness.
- Nothing here covers acoustics, thermal loss, condensation, fire or smoke requirements, or structural support of the duct.
Frequently asked questions
What is a duct friction rate?
The friction rate is the pressure the air loses per 100 feet of straight duct, expressed in inches of water gauge. It is the design constant a designer picks before sizing, so every duct in the system loses pressure at the same rate per foot.
What friction rate should I design to?
There is no universal correct value. Residential systems are commonly designed between 0.05 and 0.10 in. w.g. per 100 ft, commercial trunks between 0.08 and 0.15, and high-velocity or industrial systems at 0.20 and above. The right number depends on available fan static pressure, the noise target, space constraints and the design standard for the project.
What happens if I pick too high a friction rate?
The ducts come out smaller and cheaper to buy, but velocity, noise and fan power all rise, and the fan may not have the static pressure to deliver design airflow on the critical path.