8 guides published so far — more are added regularly.
The exact equal-friction and equivalent-diameter formulas this duct sizing calculator uses, step by step, plus assumptions and limitations. Grounded in the actual calculator code.
Duct sizing chart: round duct size, velocity and actual friction rate for common CFM values at the standard 0.1 in.wg/100 ft residential friction rate. Computed from the real formula.
How to convert round duct to an equivalent rectangular size using ASHRAE's equivalent-diameter formula. Worked conversion tables and why area comparisons undersize rectangular duct.
How to convert CFM to duct size step by step, with the exact equal-friction formula and a worked 400 CFM example, plus when to use velocity mode instead.
How to size a return air duct: why returns target lower velocity than supply branches, worked examples by system tonnage, and common return-sizing mistakes.
How much bigger flex duct needs to be than rigid galvanized duct for the same airflow, why installation quality matters more than the math, and practical sizing adjustments.
Estimate main trunk duct size from AC system tonnage using the ~400 CFM/ton rule of thumb, with a worked table from 1.5 to 5 tons and guidance on branch splitting.
The fpm thresholds this calculator uses to flag quiet, typical and too-fast duct velocity, why fast air is noisy, and how to troubleshoot an existing noisy vent.