HVAC Duct Calculator

Size HVAC ducts correctly by airflow and design velocity. Enter your CFM to get an instant round duct diameter, plus matching rectangular dimensions.

Airflow CFM From your equipment spec or load calculation Please enter a positive number

Design velocity ft/min Typical residential supply duct: 600–900 ft/min Please enter a velocity between 100–3000 ft/min

Rectangular duct height inches Used to calculate the matching rectangular width Please enter a positive number

Duct material Flex duct is sized larger to offset higher friction loss

Calculation Results

Round duct diameter

— in

Sized using the velocity method: duct area = CFM ÷ design velocity.

How to use this calculator

Enter your airflow in CFM

Pull this from your equipment's rated output or a Manual J/D load calculation.

Set your design velocity

Higher velocity means a smaller duct but more noise; lower velocity means a larger, quieter duct.

Enter duct height and material

Height gives you the matching rectangular width. Select flexible duct if that's what you're installing.

Get your instant duct size

See the round duct diameter, rectangular width, and equivalent square duct side all at once.

Required Round Duct Diameter by Airflow (CFM)

Airflow (CFM)	Target Velocity	Required Area (Sq Ft)	Round Duct Diameter
100 CFM	400 ft/min	0.25 ft²	6.8 inches
100 CFM	600 ft/min	0.17 ft²	5.5 inches
100 CFM	800 ft/min	0.13 ft²	4.8 inches
100 CFM	1000 ft/min	0.10 ft²	4.3 inches
250 CFM	400 ft/min	0.63 ft²	10.7 inches
250 CFM	600 ft/min	0.42 ft²	8.7 inches
250 CFM	800 ft/min	0.31 ft²	7.6 inches
250 CFM	1000 ft/min	0.25 ft²	6.8 inches
500 CFM	400 ft/min	1.25 ft²	15.1 inches
500 CFM	600 ft/min	0.83 ft²	12.4 inches
500 CFM	800 ft/min	0.63 ft²	10.7 inches
500 CFM	1000 ft/min	0.50 ft²	9.6 inches
1000 CFM	400 ft/min	2.50 ft²	21.4 inches
1000 CFM	600 ft/min	1.67 ft²	17.5 inches
1000 CFM	800 ft/min	1.25 ft²	15.1 inches
1000 CFM	1000 ft/min	1.00 ft²	13.5 inches
1500 CFM	400 ft/min	3.75 ft²	26.2 inches
1500 CFM	600 ft/min	2.50 ft²	21.4 inches
1500 CFM	800 ft/min	1.88 ft²	18.5 inches
1500 CFM	1000 ft/min	1.50 ft²	16.6 inches
2000 CFM	400 ft/min	5.00 ft²	30.3 inches
2000 CFM	600 ft/min	3.33 ft²	24.7 inches
2000 CFM	800 ft/min	2.50 ft²	21.4 inches
2000 CFM	1000 ft/min	2.00 ft²	19.1 inches

Frequently asked questions

What CFM should I use for duct sizing?

Use the CFM from your equipment's rated output or, ideally, a room-by-room Manual J load calculation. Individual residential branch runs typically carry 75–400 CFM, while trunk lines feeding multiple rooms carry much more.

What duct velocity should I design for?

Residential supply branches typically run 600–900 ft/min, return ducts a bit lower at 500–700 ft/min, and main trunk lines up to 900–1,200 ft/min. Going higher saves duct space but increases noise; going lower is quieter but needs larger ducts.

How do I convert a round duct size to rectangular?

Round and rectangular ducts are equivalent when their cross-sectional areas match. Enter your target duct height and this calculator solves for the width that gives the same area as the calculated round diameter.

Does flexible duct need to be sized differently than metal duct?

Yes. Flexible duct has a corrugated interior that creates significantly more friction loss than smooth rigid metal, so it's commonly sized larger for the same airflow and static pressure. This calculator applies a standard area allowance when you select flexible duct.

How accurate is this duct sizing calculator?

This is a velocity-method estimate suitable for planning and quick sizing. A full system design should follow ACCA Manual D and account for total external static pressure, friction rate per 100 ft, and fitting losses — consult an HVAC engineer for final specifications.

About this calculator

This HVAC duct calculator sizes supply and return ducts using the velocity method — the standard quick-sizing approach used by installers and designers before running a full system design. It's built for HVAC technicians, contractors, and homeowners planning a duct run.

The calculation follows the standard area-and-velocity relationship for duct sizing:

Duct area (ft²) = Airflow (CFM) ÷ Design velocity (ft/min)
Round duct diameter (in) = √(183.3 × Duct area (ft²))
Rectangular width (in) = (Duct area (ft²) × 144) ÷ Duct height (in)

This method sizes duct cross-section correctly for your target velocity, but it does not calculate total system static pressure, friction rate over duct length, or fitting and elbow losses. For a complete, code-compliant design, follow ACCA Manual D or have an HVAC professional verify the full duct run.