Duct Sizing Calculator

This calculator sizes ductwork using the equal-friction method from SMACNA's HVAC Duct Construction Standards -- the industry-standard approach of picking a single design friction rate and solving the duct sizing chart for the diameter that delivers a given airflow at that friction rate, rather than targeting a fixed velocity. Pressure Drop is simply Friction Rate echoed back -- the two are identical by definition, since Friction Rate is entered in the same "WC/100ft units this calculator reports pressure drop in, so Airflow, Duct Shape, and Aspect Ratio have zero effect on it. Round Duct Diameter comes from Airflow and Friction Rate through the equal-friction sizing formula, and Airflow has roughly twice the proportional pull on diameter that Friction Rate does -- the formula raises Airflow to the 0.378 power and Friction Rate to the -0.199 power, so a given percentage change in Airflow moves diameter about twice as much as the same percentage change in Friction Rate.

Duct Shape and Aspect Ratio have no effect on Round Duct Diameter itself; they only come into play afterward, converting that round diameter into an equivalent rectangle. At the default Round shape, Rectangular Width and Height stay at zero regardless of Aspect Ratio, since aspect ratio only has meaning once you switch Duct Shape to Rectangular -- selecting Rectangular for an 830 CFM, 0.08 "WC/100ft duct at the default 2:1 aspect ratio produces roughly a 20x10 inch duct in place of a 14 inch round one, at a lower air velocity for the same airflow (roughly three-quarters of the round duct's velocity, since the rectangular cross-section is about 30% larger). This tool implements the SMACNA equal-friction method for straight duct runs; it does not size fittings, transitions, or account for elevation changes.