Ductulator.
Give air a little room.
Compare duct sizes and friction loss.
Not a fluid 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.
Follow the air through a rooftop unit ↗
Service report builder ↗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
This is a straight-duct calculator, not a complete duct-system design. It does not determine required airflow, fitting losses, fan capacity, noise, leakage or code compliance. The sizing button uses the two stated limits as an example, not a universal design recommendation. Confirm available sizes with your fabricator.