Duct Size Calculator
Find the recommended round duct diameter for a given airflow (CFM) at a target duct velocity, a standard HVAC sizing method.
How this calculator works
This calculator finds the round duct diameter needed to carry a given airflow at a target velocity. It first converts your airflow and design velocity into a required cross-sectional area in square inches, then converts that area into a diameter, since ductwork is ordered and installed by diameter, not by area.
Formula: Required duct area (sq in) = (airflow in CFM ÷ design velocity in ft/min) × 144. Round duct diameter = √(area ÷ (π/4)).
Worked example
400 CFM at a 700 ft/min design velocity:
- Area: (400 ÷ 700) × 144 ≈ 82.29 sq in
- Diameter: √(82.29 ÷ 0.7854) ≈ 10.24 in
Notes
Supply duct velocity is typically designed around 700-900 ft/min for residential systems, balancing airflow against noise. A narrower duct than that range raises velocity and noise; a wider one than needed wastes material and takes up more space in the framing. Rectangular ducts aren’t sized directly with this round-duct formula — they use an equivalent-area conversion from the same required area instead.
How to use
Enter your airflow in CFM and a design velocity in ft/min (700-900 ft/min is a common residential starting range). The calculator returns the required duct area and the round duct diameter that delivers that airflow at your chosen velocity.
A standard sizing method for planning. Full duct design (static pressure, branch runs, equivalent lengths) should follow ACCA Manual D or be confirmed with an HVAC contractor.
Frequently asked questions
What does duct size have to do with airflow and velocity?
For a given airflow in CFM, a narrower duct forces the air to move faster to carry the same volume, and a wider duct lets it move slower. Duct sizing works backward from a target velocity — pick a velocity, and the calculator finds the duct area, then diameter, needed to hit it at your airflow.
What design velocity should I use for residential ductwork?
700-900 ft/min is the typical range used for residential supply ducts. Lower in that range keeps the system quieter; higher lets you use smaller, less bulky ductwork. Going much outside that range either wastes space and material or creates noticeable noise and pressure problems.
Can I use this formula for rectangular ducts?
Not directly. This calculator solves for a round duct diameter. Rectangular duct sizing starts from the same required area but then applies an equivalent-area conversion to find compatible rectangular dimensions, since a rectangular duct's area doesn't convert to its sides the same way a circle's area converts to its diameter.
What happens if my duct is sized too small?
An undersized duct forces air through at a higher velocity than intended, which increases static pressure, adds strain on the blower motor, and often creates noticeable air noise at registers. Sizing to a reasonable target velocity, rather than the smallest duct that physically fits, avoids these problems.
Estimates only. Verify quantities with your supplier before purchasing.