How Duct Size Is Calculated
Duct sizing starts with the airflow you need to deliver, measured in cubic feet per minute, and a target velocity appropriate for that section of ductwork. The required cross sectional area is airflow divided by velocity, and from that area this calculator solves for either a round duct diameter or, for rectangular duct, a width based on a height you specify.
Rounding to the nearest standard duct size is always recommended in the field, since ductwork is manufactured in fixed increments, not exact custom dimensions. This calculator gives you the exact theoretical size so you can round up to the nearest available standard size with confidence rather than guessing.
Choosing a Target Velocity
- 600 fpm: Very quiet, common for bedrooms or noise sensitive spaces
- 700 fpm: Standard for most residential supply trunks
- 900 fpm: Common for branch ducts feeding individual rooms
- 1200 fpm: Higher velocity, more noise, used where space is limited
Higher velocity means a smaller duct can carry the same airflow, but it also means more noise and more static pressure loss. Lower velocity means quieter operation but requires a larger duct to move the same amount of air.
Round vs Rectangular Duct
Round duct is generally more efficient, since it has less surface area for the same cross sectional area, meaning less friction loss and material cost per foot of airflow carried. Rectangular duct is often used where ceiling or wall cavity space is limited and a round duct simply will not fit. This calculator lets you check both shapes for the same airflow requirement.
Why Correct Duct Sizing Matters
Undersized ducts increase air velocity beyond comfortable noise levels and create excess static pressure, which forces the blower to work harder and can shorten equipment life. Oversized ducts waste material and can lead to poor air distribution. Getting the size right the first time avoids callbacks and comfort complaints after installation.