Duct Size Calculator
Calculate required duct dimensions based on CFM airflow requirements and maximum velocity.
Typical: 600-1200 fpm
Results
How to Use This Calculator
Enter the required airflow in cubic feet per minute (CFM) for the duct run. A typical residential supply duct might carry 100 to 200 CFM. Next, specify the maximum allowable friction rate, commonly 0.08 inches of water gauge per 100 feet of duct for residential systems. Select the duct shape: round, rectangular, or oval. For rectangular ducts, you can constrain one dimension if space is limited, such as a maximum height of 8 inches for a joist cavity. The calculator returns the recommended duct diameter for round ducts or the width and height for rectangular ducts. For example, 150 CFM at 0.08 in. w.g. per 100 feet requires approximately a 7 inch round duct or a 10 by 6 inch rectangular equivalent.
Understanding the Concept
Proper duct sizing is fundamental to HVAC system performance. Undersized ducts create excessive air velocity and static pressure, leading to noise complaints, reduced airflow at registers, and higher blower energy consumption. Oversized ducts waste material costs and can reduce air velocity below the minimum needed to keep the air stream mixed, leading to temperature stratification. The duct sizing process balances airflow volume, friction loss, and available space. ASHRAE recommends designing residential duct systems with friction rates between 0.06 and 0.10 inches of water gauge per 100 feet of equivalent duct length. Commercial systems may use higher friction rates where fan power is less of a concern. The equal friction method is the most widely used approach for simple systems, while the static regain method is preferred for large commercial supply duct systems to maintain uniform pressure at each outlet.
The Formula Explained
The equal friction method uses the Darcy Weisbach equation adapted for air ducts. The friction rate formula is: Pf = (f x L x V^2) / (2 x D x g), where Pf is pressure loss, f is the friction factor, L is duct length, V is velocity, D is hydraulic diameter, and g is gravitational acceleration. In practice, ASHRAE Duct Fitting Database and friction charts simplify this. For round ducts, the relationship between CFM, diameter, and friction rate is: D = 0.109 x (Q^0.4) x (Pf^(-0.2)), where Q is airflow in CFM, D is diameter in inches, and Pf is friction rate in inches w.g. per 100 feet. Rectangular equivalent diameter is calculated using: De = 1.3 x ((a x b)^0.625) / ((a + b)^0.25), per ASHRAE Handbook of Fundamentals Chapter 21.
Frequently Asked Questions
What size duct do I need for 400 CFM?
At a standard friction rate of 0.08 inches of water gauge per 100 feet, 400 CFM requires a 10 inch round duct. The rectangular equivalent would be approximately 12 by 8 inches or 14 by 7 inches. Always verify with the actual friction rate specified in your system design.
What is the maximum air velocity in residential ductwork?
ASHRAE recommends maximum air velocities of 700 to 900 feet per minute in residential main ducts and 500 to 700 FPM in branch ducts. Velocities above these levels cause noticeable noise at supply registers. Flex duct should be kept below 600 FPM due to its higher friction rate.
How do I convert a round duct size to rectangular?
Use the equivalent diameter formula: De = 1.3 x ((W x H)^0.625) / ((W + H)^0.25). For example, a 10 inch round duct is equivalent to an 11 by 8 inch rectangular duct, a 13 by 7 inch duct, or a 16 by 6 inch duct. The rectangular duct must match the round duct's equivalent diameter, not just cross sectional area.
Does flex duct require a larger size than sheet metal duct?
Yes. Fully stretched flex duct has roughly 1.5 to 2 times the friction rate of smooth sheet metal duct. When using flex duct, increase the duct diameter by one size, going from 6 inch to 7 inch, for example. Flex duct that is not fully stretched can have four times the friction rate of metal duct.
What friction rate should I use for duct sizing?
Use 0.08 inches of water gauge per 100 feet for most residential systems. This provides a good balance between duct size, noise, and fan energy. For low velocity commercial systems, use 0.06 to 0.08. High velocity commercial systems may use 0.15 to 0.40 in. w.g. per 100 feet.