Engineering Calculators

Duct Sizing Calculator

Airflow & Conditions

Affects air density via standard atmosphere correction.

Friction Factor Method

Colebrook-White solves the implicit friction-factor equation by iterating to a converged answer — more exact, but takes marginally longer. Swamee-Jain is a direct (non-iterative) approximation of Colebrook-White, accurate to within about 1% for typical turbulent flow.

Duct Shape

Sizing Method

Formula Reference

Diameter from Target Friction Rate

Equal Friction sizing has no closed-form diameter formula — the diameter is solved iteratively via the Darcy-Weisbach equation until the actual friction rate matches your target.

D = round duct diameter, Q = airflow, V = velocity, D_e = equivalent diameter, a/b = rectangular duct side lengths.