semkuDuct Calculator
Estimate round and rectangular straight-duct sizes from airflow, velocity and friction limits, or convert equivalent sizes.
Round diameter:
Actual mean velocity:
Estimated straight-duct friction:
Rectangular sides:
Actual mean velocity:
Estimated straight-duct friction:
Equal-friction diameter:
Sizes and diameters are rounded to 2 decimal places, velocity to 1 and friction to 4.
Standard-air, galvanized straight-duct estimate. Excludes fittings, equipment, leakage and noise; professional design review is required for installation.
Calculation rules
Units: 1 ft = 0.3048 m, 1 in = 0.0254 m, 1 in. w.g. = 249.08891 Pa. Standard air: density 1.204 kg/m³, kinematic viscosity 1.5 × 10⁻⁵ m²/s; galvanized roughness ε = 0.09 mm.
Velocity V = airflow ÷ actual area (round πD²/4, rectangular ab). Friction uses the hydraulic diameter Dh (round D, rectangular 2ab/(a + b)) and Re = V·Dh/ν.
Re below 2300: f = 64/Re. Re from 2300 to below 4000: friction is not shown. Re 4000 and above: Colebrook, 1/√f = −2 log₁₀(ε/(3.7 Dh) + 2.51/(Re √f)). Friction rate = f·ρ·V²/(2 Dh), converted to in. w.g. per 100 ft.
Equal-friction diameter (Huebscher): De = 1.30 (ab)^0.625 / (a + b)^0.25. The reverse uses b = D (r + 1)^0.25 / (1.30 r^0.625) and a = r·b. It is not an equal-area conversion.
Sizing checks whole-inch candidates from 2 to 120 in: the smallest round diameter, and for rectangles each short side b with a = ⌈r·b⌉ ≤ 120 in, keeping the smallest area (on a tie, the smaller a). A candidate must meet both limits; these are calculation candidates, not a list of stocked sizes.
Results are rounded once at display, with an exact halfway value rounded up.
Sources and definitions
ASHRAE Handbook—Fundamentals (2017), Chapter 21: Duct Design
Source for the Darcy and Colebrook relations, hydraulic diameter, the Huebscher equivalent diameter, galvanized roughness and standard air density.
NIST Guide to the SI, Appendix B: Conversion Factors
Source for the inch and foot definitions.
Frequently asked questions
- Does sizing check both limits?
- Yes. A candidate must meet both maximum velocity and maximum straight-duct friction.
- Is the equivalent diameter based on equal area?
- No. The Huebscher relation approximates equal friction at equal flow and length.
- Why can two velocities differ?
- Actual velocity depends on the actual cross-sectional area. Equal-friction circular and rectangular sections need not have equal areas.
- Does this select a fan?
- No. Fittings, equipment and the complete duct network must also be considered.
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Calculations run in your browser without sending your calculation inputs or results to the server. See “Privacy on this page” for information about access data.