Williams Air Solutions

Duct Sizing Calculator

What size ductwork do I need for this airflow? Undersized ducts restrict airflow and shorten equipment life. Oversized ducts waste material and space.

Airflow

CFM
Airflow the duct needs to deliver. Use the CFM Calculator if you do not know this number.
FPM
Residential supply: 600 FPM. Branch: 400-600 FPM. Return: 500-700 FPM. Commercial: 1,000-2,000 FPM.

Duct Type

Sets the default target velocity. Override the velocity field above if you have a specific requirement.
inches
If you need rectangular duct and one dimension is fixed (e.g., 8" height for a soffit), enter it here. The calculator will find the other dimension.

Enter a CFM value to calculate the required duct size.

What to do with this number
  1. Round up to the next standard size. Duct is sold in standard diameters (6", 7", 8", 10", 12", 14"). If the calculation says 9.2", use 10". Undersizing restricts airflow and kills comfort.
  2. Check available space. Round duct is most efficient but rectangular may fit where round will not. A 10" round has the same area as an 8×12 rectangle. Use rectangular in tight cavities, soffits, and between joists.
  3. Watch velocity for noise. Keep branch ducts under 600 FPM for bedrooms and living spaces. Trunk lines can run 700–900 FPM. Velocity above 900 FPM causes audible air noise.
  4. Verify total system CFM. All branch ducts together must not exceed the air handler or furnace CFM rating. Use the CFM Calculator to check individual room requirements.

These calculations use ACCA Manual D velocity method and ASHRAE duct design standards. Verify with a full duct design for permit-required installations.

Related Topics

CFM Calculator
Sources (3)
  • ACCA Manual D — duct sizing methodology
  • ASHRAE Fundamentals Ch. 21 — duct design
  • Engineering ToolBox — duct velocity tables