Skip to content
Omnikalc

Duct size calculator: round duct diameter from airflow

Find the round duct diameter for an airflow and a target air velocity, the nearest standard size and the velocity it gives, plus a rectangular equivalent.

Updated on
Your inputs
CFM

Value out of range

ft/min

Value out of range

in

Value out of range

Results

Required round duct diameter

10.2 in

Standard size to use

12 in

Air velocity in the standard size
509 ft/min
Quiet range for most residential ducts
Cross-sectional area
82.3 sq in
Width of a rectangular duct with the same area
10.3 in

Why duct size matters

Ducts carry heated or cooled air from the furnace or air handler to each room. Undersized ducts force air to move too fast: the system gets noisy, the fan works harder, and rooms at the end of the run get too little air. Oversized ducts cost more and take up space. Sizing starts from two numbers: the airflow the room or branch needs and a target velocity.

The formula

Area = airflow ÷ velocity, then for a round duct diameter = √(4 × area ÷ π)

With airflow in CFM and velocity in feet per minute, the area comes out in square feet; multiply by 144 for square inches. The calculator rounds up to the next standard round duct size and shows the actual velocity at that size, plus the width of a rectangular duct with the same area for a chosen height.

Worked example

A trunk line must carry 400 CFM at a target of 700 ft/min.

  • Area = 400 ÷ 700 = 0.571 sq ft = 82.3 sq in
  • Diameter = √(4 × 82.3 ÷ π) ≈ 10.2 in
  • Standard size: 12 in, which gives an actual velocity of about 509 ft/min
  • An 8-inch-high rectangular duct with the same area would be about 10.3 in wide

Typical design velocities (residential)

Duct Velocity
Supply trunk 700–900 ft/min
Supply branch 500–600 ft/min
Return trunk 600–700 ft/min
Return branch 400–500 ft/min

Lower velocities mean quieter systems; commercial systems often run at 1,000 to 2,000 ft/min.

How much airflow?

  • A rough rule is 400 CFM per ton of air conditioning capacity.
  • For rooms, about 1 CFM per square foot is a common starting point.
  • The proper method is an ACCA Manual J load calculation for each room, followed by Manual D duct design.

Round versus rectangular

At the same cross-sectional area, a rectangular duct has more surface and more friction than a round one, especially when it is flat. Friction-based “equivalent diameter” charts give a slightly larger rectangular size than the equal-area width shown here. Keep the aspect ratio below about 4:1 where possible.

Other tips

  • Flexible duct has much more friction than metal; keep runs short and fully stretched.
  • Seal joints with mastic or approved tape: leaky ducts can lose 20% to 30% of the air.
  • Insulate ducts that run through unconditioned attics or crawl spaces.
  • Balance the system with dampers after installation.

Frequently asked questions

How do you size a round duct?

Divide the airflow by the target velocity to get the area, then find the diameter: d = √(4 × area ÷ π). 400 CFM at 700 ft/min needs 0.57 sq ft, a diameter of about 10.2 inches, so use a 12-inch duct.

What air velocity should I use?

In homes, about 700 to 900 ft/min in supply trunks and 500 to 600 ft/min in branches keep noise low. Commercial systems often run faster.

How much airflow does a room need?

A common starting point is about 1 CFM per square foot of floor, or 400 CFM per ton of air conditioning. A Manual J load calculation gives the real figure.

Is a rectangular duct of the same area equivalent?

Only roughly. A flat rectangular duct has more friction than a round duct of the same area, so the equivalent rectangular size is usually a little larger. Use a duct calculator or friction chart for final sizing.

Was this calculator helpful?

Thanks for your feedback!

Results are estimates provided for information purposes. They do not replace professional advice.

Related calculators