HVAC & Ventilation: Calculators, Diagrams & Guides

Ventilation is the quiet half of a healthy home, and it is where snippet answers go wrong most often. A single number pulled from a search result usually confuses the code-required minimum with the best-practice recommendation, and almost never mentions that a fan rated in a lab moves far less air through a real duct. The calculators here separate those ideas on purpose: each one shows the flat minimum your code demands next to the size that actually fits your room, and then tells you how to buy a unit that still delivers once it is fighting a duct run.

The bathroom exhaust fan calculator is the anchor of this section. It reports the IRC and ASHRAE 62.2 floor of 50 CFM intermittent or 20 CFM continuous, layers on the Home Ventilating Institute methods that scale the fan to your square footage, ceiling height, and fixtures, and applies the delivered-versus-rated derating so the fan you install clears the mildew instead of just spinning. As the section grows it will add range-hood CFM with the makeup-air trigger, whole-house ventilation rate where the ASHRAE and IRC formulas genuinely disagree, ceiling-fan blade span, and dehumidifier capacity on the post-2019 test scale.

Everything is keyed to the current standards — ASHRAE 62.2-2019, the 2021 and 2024 International Residential Code mechanical chapters, and the manufacturers’ own delivered-airflow certifications — and stays materials-and-sizing only, with no pricing. Each calculator is free with no signup, cites the exact code section it rests on, and calls out where a state amendment such as California Title 24 changes the answer, so you can size a fan or a vent once and pass inspection the first time.