What velocity should I design for?

Residential supply trunks target 700–900 FPM; branch runs are often 600–700 FPM to keep noise low.

Is round or rectangular duct better?

Round duct moves air most efficiently for a given area, but rectangular duct fits tight framing better.

HVAC Duct Calculator

Size supply ducts for a target airflow in CFM.

100% free to useNo sign-up requiredImperial & metric units

What size duct do I need for the airflow?

Duct size comes from airflow divided by target velocity. At a quiet 800 FPM, 400 CFM needs a cross-section of 72 square inches, which is about a 10 inch round duct. Keep supply runs between 700 and 900 FPM to balance noise against duct size, then round up to the next standard size.

Inputs

Required airflowCFM

Target velocityFPM

Lower is quieter; 700-900 FPM suits most supply runs.

Duct shape

Results

Round Duct

10 in

Round up to standard size

Exact Diameter

9.57 in

Cross Section

72 sq in

Air Velocity

800 FPM

Estimates update instantly as you type. Confirm against local code before ordering materials.

Undersized ducts are the hidden cause of noisy, uneven, and inefficient HVAC systems. This calculator sizes a duct from the airflow it must carry and the target air velocity, returning the round duct diameter, the exact diameter before rounding, the cross-sectional area, and the equivalent square duct dimension.

Velocity is the lever that balances noise against duct size: lower velocity is quieter but needs a larger duct, while higher velocity saves space at the cost of noise and friction. Use the result as a starting size, then confirm against a full Manual D design for the complete system.

How duct size is calculated

Duct size comes from the required airflow (CFM) and a target air velocity. The cross-sectional area equals CFM divided by velocity, which converts to a round duct diameter.

Area (sq ft) = CFM ÷ Velocity (FPM); Diameter = √(Area ÷ 0.7854) × 12

Round duct size by airflow

Approximate round duct diameter at about 800 FPM velocity.

Airflow	Cross section	Round duct
100 CFM	18 sq in	5 in
200 CFM	36 sq in	7 in
400 CFM	72 sq in	10 in
800 CFM	144 sq in	14 in
1,200 CFM	216 sq in	17 in

These are starting sizes; confirm against a full Manual D design for the complete system.

How to use it

1Enter the required airflow for the run in CFM.
2Set the target air velocity in feet per minute.
3Review the recommended round duct diameter.
4Round up to the nearest standard duct size.

Key terms explained

CFM: Cubic feet per minute, the volume of air the duct must move to condition a space.
FPM: Feet per minute, the air velocity inside the duct. Lower velocity is quieter.
Cross-sectional area: The open area of the duct, equal to airflow divided by velocity.
Equivalent diameter: The round duct size that carries the same airflow as a given rectangular duct.

Worked examples

Bedroom supply

100 CFM at 700 FPM calls for about a 5 inch round duct.

Main trunk

800 CFM at 900 FPM needs roughly a 13 inch round duct.

Pro tips from the field

1Keep supply trunk velocity around 700 to 900 FPM in homes to limit noise.
2Always round up to the next standard duct size; rounding down chokes airflow.
3Rectangular ducts need a larger area than round for the same airflow due to friction.
4This sizes a single run, a full system needs a Manual D calculation for balance.

Common mistakes

Designing for too high a velocity, which creates noise.
Sizing flex duct the same as smooth metal duct.
Ignoring total static pressure across the system.

Where it gets used

Branch sizing

Quickly size a supply or return branch to a room from its CFM target.

Retrofit checks

Verify whether existing ducts can carry a larger system before upgrading.

Design starting point

Set initial duct sizes before a full load and distribution calculation.

Frequently asked questions