Duct Sizing Calculator
A duct sizing calculator determines the required dimensions of HVAC supply and return ducts based on the required airflow in CFM and the design air velocity or friction rate. Correctly sized ductwork ensures adequate airflow to all conditioned spaces, minimizes fan energy consumption, and avoids excessive noise from high-velocity air. This tool calculates the required cross-sectional area from the CFM and velocity inputs, then outputs the diameter for a round duct and suggests an equivalent rectangular duct size using the SMACNA hydraulic diameter method. Enter the airflow, design velocity, and select the duct type.
Duct sizing formula
Required Area (sq ft) = CFM / Velocity (fpm)
Required Area (sq in) = Area (sq ft) x 144
Round diameter (in) = sqrt(4 x Area (sq in) / pi)
Rectangular height (in) = Area (sq in) / Width (in) [for given width]
Standard round duct sizes are available in 2-inch increments. Always round up to the next standard size and recalculate the actual velocity at the selected size.
Frequently asked questions
How do I size an HVAC duct?
Use the equal friction method: select a design friction rate (typically 0.08 to 0.10 in wg per 100 ft for residential and light commercial), then find the duct diameter or dimensions that convey the required CFM at that friction rate. ASHRAE duct design charts or the Darcy-Weisbach equation with Colebrook friction factor are used.
What is the recommended duct velocity for HVAC?
Recommended velocities: supply main ducts = 700-900 fpm, supply branch ducts = 500-700 fpm, return air main = 600-800 fpm, residential supply = 400-600 fpm. Higher velocities increase pressure loss and noise; lower velocities require larger ducts. ASHRAE Fundamentals provides detailed guidance.
What is the equivalent diameter of a rectangular duct?
The hydraulic diameter of a rectangular duct is De = 1.30 x (a x b)^0.625 / (a + b)^0.25, where a and b are the duct dimensions in inches. This is the SMACNA-recommended formula for converting rectangular duct sizes to equivalent round duct diameters for friction loss calculations.
What is the friction rate for residential ductwork?
For residential systems, ACCA Manual D recommends designing for a total available static pressure (TASP) of typically 0.5 to 0.8 inches of water gauge (in wg), with friction rates of 0.05 to 0.10 in wg per 100 feet of equivalent duct length depending on the system.
How does duct area relate to CFM and velocity?
CFM = Area (sq ft) x Velocity (fpm). To find the required duct area, divide CFM by the design velocity in feet per minute (fpm). Convert to square inches for the duct dimensions. For a round duct, the diameter is D = sqrt(4 x Area / pi) in inches.
Official sources
- ASHRAE: ASHRAE Handbook - HVAC Systems and Equipment (Duct Design chapter).
- SMACNA: SMACNA HVAC Duct Construction Standards - Metal and Flexible.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.