DuctCalc What is friction rate in ductwork, and what does a ductulator do with it?
Friction rate is the pressure a duct loses per unit length, in inches of water per 100 ft or in pascals per metre. A ductulator takes an airflow and a friction rate and returns the duct size; DuctCalc does the same arithmetic as the cardboard wheel.
A ductulator is the wheel, chart or app that links four things: airflow, friction rate, velocity and diameter. Fix two and the other two follow. DuctCalc lets you fix airflow plus friction, or airflow plus velocity, and shows what the others turn out to be.
Why 0.1?
Under Reference > Friction rate, briefly, the app lists the usual design points: 0.08 to 0.10 in.wg/100 ft for supply ducts (0.65 to 0.8 Pa/m) and 0.05 to 0.08 for returns (0.4 to 0.65). Size every run to one rate and the system roughly balances itself. That is why the Friction rate field starts on 0.1, or on 1 Pa/m in metric.
What a given airflow does to the rate
The same 400 CFM through galvanized round ducts of different diameters:
| Diameter | Velocity | Friction rate |
|---|---|---|
| 8 in | 1,146 fpm | 0.256 |
| 9 in | 905 fpm | 0.144 |
| 10 in | 733 fpm | 0.086 |
| 12 in | 509 fpm | 0.035 |
| 14 in | 374 fpm | 0.017 |
Going from 10 in to 8 in triples the friction. That steepness is why a ductulator is worth using: a small change in diameter moves the pressure a lot.
How does it calculate?
The method is Darcy-Weisbach with the Colebrook friction factor, for standard air at 20 °C at sea level. You can read the model names on the last lines of Settings. Rectangular duct goes through the Huebscher equivalent diameter. Nothing else is hidden: no lookup table and no curve fit.
A metric example
- Set Settings > Units > Unit system to Metric and pick L/s in Airflow unit.
- In Size a duct type 200 in Airflow. Friction rate starts on 1 Pa/m.
- Required round duct shows 241 mm. Use stock duct size says 250 mm at 4.1 m/s and 0.84 Pa/m.
- The rectangular list opens with 250 x 200 mm at 4.0 m/s, 350 x 150 and 600 x 100.
The metric stock sizes follow EN 1506: 80, 100, 125, 160, 200, 250, 315 mm and on up to 1,250.
Questions and answers
What friction rate should I design to?
What is a ductulator app?
Is friction rate the same as static pressure?
More DuctCalc guides
- How to calculate duct size from CFM
- Round to rectangular duct equivalent, with real velocities
- Recommended duct velocity: the chart, and checking a real duct
- Flex duct friction loss compared with galvanized and duct board
- CFM per ton: the chart, and the duct size that carries it
- How many CFM can a 6 inch duct carry?
- All questions about DuctCalc
- Back to DuctCalc