Ductulator.
Compare duct sizes, air velocity and friction loss for straight galvanized duct.
not a simulation.
At or below 0.08 in. w.g./100 ft
and 1,500 FPM. Whole-inch sizes.
Volume flow ÷ cross-sectional area
Straight duct only, excluding fittings
20 × 12 inch rectangular duct
Change a dimension to see the tradeoff.
Choosing a size
A smaller duct takes less space, but carries the same airflow at higher velocity and friction loss.
Air conditions
Airflow is actual volume at these conditions. Changing air density affects friction loss, not velocity at a fixed volume flow.
How the calculation works
Friction loss uses Darcy–Weisbach with galvanized-steel roughness and an air-density correction. Rectangular duct uses hydraulic diameter, an approximation rather than an equal-friction round conversion. Read the method, sources and limits.
Limitations
It sizes straight duct only. It doesn’t work out required airflow, fitting losses, fan capacity, noise, leakage or code compliance. The round-size button uses the two limits shown under it as an example; set your own for the job. Check available sizes with your fabricator.
Software for the rest of the service day
BoltWright is commercial HVAC service software with AI built in. Plan the visit, pull up the equipment history, and turn the crew’s notes and photos into reports your office can review.