TOOL GUIDE
Duct sizing
Size or check round/rectangular ducts against airflow, velocity and friction-loss rate.
Purpose & scope
Size or check round/rectangular ducts against airflow, velocity and friction-loss rate.
Straight-duct losses only. Hydraulic diameter is used for friction; equivalent round diameter is for comparison. At most 10 candidates are retained.
Inputs
Use drawings, design conditions and manufacturer data. Units and example values follow the calculator; example values are not universal design requirements.
| Parameter | Value / interpretation |
|---|---|
| Design preset | Select the material/system appropriate to the actual application; verify reference values. |
Steps
- Use Size selection for candidates or Check duct for an existing section.
- Enter airflow, units, material, temperature and project design limits.
- Read velocity, Pa/m and section loss; choose a table row and save it in the project list.
Worked example
This reproducible training example uses the values below. Select the appropriate mode and overwrite restored project values. Example rates, K values and equipment losses are assumptions, not mandatory design criteria.
Examples use a decimal point and no thousands separator.
All example inputs
| Parameter | Value / interpretation |
|---|---|
| Flow | 2400 |
| Unit | m³/h |
| Width | 400 mm |
| Height | 300 mm |
| Temperature | 20 °C |
| Roughness | 0.09 mm |
| Length | 10 m |
| Maximum velocity | 6 m/s |
| Maximum friction rate | 1 Pa/m |
Use check mode for the main example, not automatic size selection. The water example first uses the pressurized chilled-water preset; its second example uses gravity drainage.
Expected results (rounded)
| Parameter | Value / interpretation |
|---|---|
| Velocity | 5.56 m/s |
| Friction loss rate | 1.01 Pa/m |
| Section loss | 10.09 Pa |
Small differences in displayed rounding are normal. A result that passes the entered limits does not establish suitability for the complete system.
Method & limitations
Straight-duct losses only. Hydraulic diameter is used for friction; equivalent round diameter is for comparison. At most 10 candidates are retained.
V = Q/A; Dh = 4A/P; Re = ρVDh/μ. Friction loss R = fρV²/(2Dh). The Darcy friction factor is solved iteratively with Colebrook for turbulent flow. Laminar flow uses f = 64/Re (an approximation for rectangular ducts); Re between 2,300 and 4,000 triggers a transitional-flow warning.
For rectangular ducts, De = 1.30(ab)^0.625/(a+b)^0.25. De is for equivalent round duct comparison only; friction calculations use hydraulic diameter and actual velocity. Air density follows the ideal gas law and viscosity follows Sutherland’s law. Up to 30 qualifying sizes closest to the maximum friction loss are retained. Dimensions range from 100 to 2,000 mm in 50 mm increments, with an aspect ratio no greater than 4:1.
Velocity and friction limits are user-defined design inputs, not universal standards. The standard and smoke extract presets reproduce the supplied sample settings. Results cover straight duct friction and exclude elbows, tees, terminals, equipment and other local losses. ASHRAE — Duct Design reference.
Common mistakes
Confusing CFM and m³/h; treating Pa/m as total pressure loss; summing parallel branches; assuming a recommended size verifies noise or outlet velocity.
Projects & reports
- Choose or create an engineering project and enter your actual design inputs.
- Project inputs/results are autosaved in this browser. Where a saved-list panel is available, name and add the selected section or alternative to compare multiple options.
- Review the report before exporting PDF. In the print dialog select Save as PDF. Export JSON for backup and use Open file to restore it; PDF cannot restore project data.
Saved results do not automatically recalculate when another tool changes. Review any dependency warning, recalculate and update the affected results.
References & related tools
The tool implements the methods and limits stated above; linking a standard does not establish complete compliance with that standard.