How to use the duct size calculator
- Enter the airflow the duct must carry, in CFM (or L/s).
- Enter the design air velocity in feet per minute (or m/s). Lower velocity means a larger, quieter duct.
- Optionally enter a height for a rectangular duct to get its width.
This is the velocity method, a quick way to estimate size. Full residential duct design follows ACCA Manual D, which sizes ducts from friction rates and the blower's available pressure. Decoratoradvice uses the velocity method here because it gives a quick, reasonable estimate for a single run.
Duct size formulas
Cross-section area = airflow ÷ velocity
Round diameter = √(4 × area ÷ π)
Rectangular equivalent (Huebscher): 1.30 × (a × b)^0.625 ÷ (a + b)^0.25 = round diameter
The rectangular size is the one with the same friction loss as the round duct, not the same area. That is why it is larger in area.
Worked example
A branch carrying 400 CFM at 700 ft/min:
Area: 400 ÷ 700 = 0.571 sq ft = 82.3 sq in
Diameter: √(4 × 82.3 ÷ π) = 10.2 in
Next common round size: 12 in
With an 8 in high rectangular duct: 8 × 11 in
In metric, 190 L/s at 3.5 m/s gives 543 cm², a 263 mm round duct; the next common size in the calculator's list is 315 mm.
Tips
- Round up to the next available size; rounding down raises velocity and noise.
- Keep runs short and use smooth bends; every fitting adds resistance.
- Seal and insulate ducts in unconditioned spaces such as attics. The insulation calculator plans the attic itself.
- Cooling capacity for a room unit comes from the BTU calculator.
Common mistakes
- Equal area instead of equal friction for rectangular ducts.
- Using one velocity for every part of the system. Main trunks and branches are often designed differently.
- Treating this as a full design. It does not check the blower's static pressure; Manual D does.
Frequently asked questions
What size duct do I need for 400 CFM?
At 700 ft/min, 10.2 in round, so a 12 in duct from the common sizes. At a lower velocity, the duct gets larger.
How do I convert a round duct to rectangular?
Use the Huebscher equivalent-diameter formula, which this calculator solves for the width when you enter a height.
What velocity should I use?
It depends on the duct's role and noise limits. Use the value from your design or ask an HVAC designer; ACCA Manual D sets out the full method.
What size duct for 100 CFM?
At 600 ft/min, 24 sq in: a 5.5 in round duct, so 6 in.
Does duct insulation change the size?
No. Size is set by the inside dimensions. Insulation wraps the outside.
How much does it cost to run the fan?
Use the blower's wattage in the electricity cost calculator.
Key takeaways
- Area = airflow ÷ velocity; diameter = √(4 × area ÷ π).
- Rectangular ducts are sized for equal friction, not equal area.
- Round up to the next common size.
- Full system design follows ACCA Manual D.