HVAC duct sizing guide

The core idea

A duct has one job: carry a known airflow without losing too much pressure or moving the air too fast. Those two limits — pressure loss and velocity — are what every sizing method balances. Give the airflow in cubic feet per minute (CFM) and a design target, and the size follows.

Friction rate vs velocity

There are two ways to set the target. Sizing by velocity is direct: velocity (ft/min) = 183.3 × CFM ÷ diameter², so for a chosen velocity the diameter is √(183.3 × CFM ÷ velocity). It's quick and intuitive, and it's how you keep a duct quiet. Sizing by friction rate is the rigorous method a full design uses: you fix how much pressure the duct may lose per 100 feet and solve for the diameter that loses exactly that much, using the Darcy-Weisbach friction relationship on the ASHRAE chart basis.

The friction rate is the input people most often get wrong, because they treat 0.10 as a universal constant. It isn't. Manual D derives it as (Available Static Pressure ÷ Total Effective Length) × 100, and since fittings dominate the effective length, the right value is often lower than 0.10. The friction rate chart has a helper for that calculation.

Round, rectangular, and flat-oval

Round duct is the most efficient shape, but cavities and joists often demand rectangular or flat-oval. To swap shapes without changing the run's pressure drop, you match the equivalent diameter: the Huebscher relationship for rectangular duct and the Heyt & Diaz correlation for flat-oval. Both match friction and capacity — but not velocity. A rectangular duct of the same equivalent diameter runs faster than the round one it replaces, so always check the actual velocity, and keep rectangular aspect ratios under about 4 to 1.

Material matters

The numbers above assume smooth galvanized steel. Flexible duct has a corrugated interior that loses two to three times the friction at the same diameter — far more when it sags or is compressed — so flex generally has to step up at least one size to carry the same air. Fiberglass ductboard sits in between. Pick the material in the console and the effective roughness changes with it.

A worked example

Say you need to move 800 CFM through a residential supply trunk. A 14-inch round duct runs at about 750 ft/min (183.3 × 800 ÷ 196 = 748) and loses roughly 0.06 in. w.c. per 100 ft — comfortably inside the 700–900 ft/min and 0.08–0.10 residential targets, a quiet and slightly conservative choice. Drop to a 12-inch duct and velocity climbs to about 1,018 ft/min with a sharply higher friction rate: smaller and cheaper, but louder and near the practical limit. That trade — one size up for quiet, one size down for tight spaces — is the everyday duct-sizing decision, and the console shows both sides of it at once.

Where the method stops

This tool sizes straight, clean galvanized round duct on the standard-air basis. It does not model fittings, the coil and filter, register losses, the blower curve, zoning, or your local mechanical code — all of which a complete ACCA Manual D design includes. Use it to estimate and sanity-check; verify with a full design, and a licensed mechanical engineer where one is required, before fabrication. The engineering disclaimer spells out every assumption.