Lucas' Ductulator

Drag the coloured disc to change duct size. Drag either white CFM ring to set airflow. ← → size, ↑ ↓ airflow, hold Shift for bigger steps.

Airflow CFM Round duct inches diameter Velocity FPM Friction in. w.g. per 100 ft

Type any value and press Enter. The wheel turns to match, keeping the airflow fixed.

When airflow changes, keep the same

Duct size Friction Velocity

Duct material Snap to standard round sizes

Air conditions

Altitude ft Air temperature °F

Velocity pressure
Wheel scale reads
Reynolds number

Rectangular equivalents

× in.

Size, in.	Velocity	Exact side

Sides rounded up to the next inch. Tap a row to load it above.

How the numbers are worked out

Velocity is airflow divided by duct area. Friction uses the Darcy–Weisbach equation with the Colebrook friction factor, using the air density and viscosity for the altitude and temperature you set, then the material factor. Rectangular sizes use the Huebscher equivalent diameter, 1.30·(ab)^0.625/(a+b)^0.25.

Fitting pressure loss 0.000in. w.g.

Velocity FPM Coefficient C, loss ÷ velocity pressure

Fitting

Fittings in this run

Fitting	Loss
Total	0.000

Each loss is saved with the velocity it was added at, so you can change the velocity between fittings.

Coefficients are transcribed from the fitting table on your SMACNA card. Check them against the printed card before relying on them. For branch fittings, C applies to the velocity the card's table names. C = 1 on the wheel reads velocity pressure.