Duct design check
Duct Velocity Calculator
Calculate face velocity in FPM from assigned CFM and either round or rectangular inside dimensions.
Round duct velocity
Use the actual inside diameter of the selected duct.
Rectangular duct velocity
Use the actual inside width and height of the selected duct.
How to use
- Assign CFM to the individual duct run.
- Enter the actual selected inside dimensions, not only a theoretical size.
- Use velocity as one check alongside pressure loss, fittings, noise, and distribution.
Formula
Round area = π × (diameter in ft)² ÷ 4
Rectangular area = width (in) × height (in) ÷ 144
Example
600 CFM in a 14 in round duct has an area of about 1.069 ft², giving approximately 561 FPM. The same CFM in an 18 in × 8 in rectangle has 1.00 ft² face area, giving 600 FPM.
Result interpretation
Velocity is a geometry and airflow relationship, not a stand-alone approval. Higher or lower velocity can affect pressure loss, sound, distribution, space, and cost. Evaluate the complete critical path and use the applicable project design criteria.
Check resistance separately
Use the Duct Friction Loss Calculator for a stated straight-run input set. It does not replace fitting, equipment, terminal, or total static-pressure calculations.
Frequently asked questions
Does this calculate pressure loss?
No. Pressure loss also depends on duct length, fittings, surface, air properties, components, and the full air path.
What velocity should I use?
This tool does not prescribe one. Select and document a velocity basis appropriate to the system, acoustic objective, pressure budget, and applicable design method.