Duct sizing calculator
Size round duct by airflow and friction rate or check an existing round or rectangular section. Calculations run entirely in this browser.
Enter design criteria and size the duct.
Transparent equations, stated assumptions.
Straight-duct pressure loss uses Darcy–Weisbach with laminar 64/Re behavior and the explicit Haaland turbulent approximation. Rectangular equivalents use the equal-friction Huebscher relation. No proprietary handbook tables or calculator source code are copied. The public Darcy–Weisbach/Colebrook basis is documented in NIST Technical Note 1887 Rev. 1.
- Standard air density: 0.075 lbm/ft³
- Dynamic viscosity: 0.0432 lbm/(ft·h)
- Galvanized duct roughness: 0.0005 ft
- Rectangular aspect ratio: 4:1 maximum
Preliminary sizing only. This tool does not replace project criteria, engineering judgment, applicable code, or current SMACNA and ASHRAE guidance. Verify air density, material roughness, pressure class, fittings, system effect, sound, leakage, and constructability before fabrication or installation.