What this calculator does
Every fan has to overcome duct friction, and the loss grows steeply with diameter, so size matters enormously. This calculator gives the velocity, Reynolds number, friction factor and pressure drop per 100 ft for a round duct, which is the first check before selecting a fan.
How to use it
- Enter the airflow and the nominal duct diameter.
- Enter the total run length so the total pressure drop can be reported.
- Use 0.0003 ft for galvanised steel duct and about 0.003 ft for flexible duct.
- Keep the friction factor in a sensible band — below 0.02 for small ducts, below 0.015 for large.
Governing formula
f = 0.25 / [log₁₀(ε/3.7D + 5.74/Re⁰·⁹)]² ; ΔP = f · (L/D) · ρV²/2 Where:
-
ε= Absolute surface roughness [ft] -
D= Internal diameter [ft] -
Re= Reynolds number of the air stream [–]
Worked example — 2,000 CFM through a 16 in galvanised duct
The figures below were produced by running this page's own engine with the example inputs (Airflow (CFM), Duct diameter (in), Total length (ft), Surface roughness (ft)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Velocity | 1432 fpm |
|---|---|
| Friction factor | 0.02 – |
| Pressure drop per 100 ft | 5.33 in.w.g. |
| Total pressure drop | 5.33 in.w.g. |
| Reynolds number | 201845 |
| Basis | Swamee–Jain friction coefficient for galvanized round duct (0.0003 ft roughness). Flexible duct needs a roughness ~0.003 ft and 1.25× diameter allowance for the equivalent length note. |
- The flow area of a 16 in duct is 1.40 ft², giving about 1,432 fpm.
- At that velocity the friction factor lands near 0.0155, so expect roughly 0.10 in.w.g. per 100 ft.
- Multiply by the real route length and add fittings before sizing the fan.
Assumptions & limitations
Engineering calculation. Evaluates the friction factor with the Swamee–Jain approximation and applies the Darcy–Weisbach relationship for standard air. Fitting and fitting losses are excluded.
- Round ducts only; rectangular ducts need the hydraulic diameter and a different friction reference.
- Fittings, elbows and take-offs are excluded — add their equivalent length or loss coefficient separately.
- Standard air only; at altitude the density and kinematic viscosity both change.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- ASHRAE Handbook — Fundamentals, duct design and fitting loss data.
- SMACNA — Engineering Manual for duct design.