Duct Size & Air Velocity Calculator
Free HVAC duct calculator. Find air velocity (FPM) from CFM and duct size for round or rectangular ducts, plus equivalent round diameter, with ACCA Manual D target velocity ranges.
How the duct velocity calculation works
Air velocity through a duct is simply the airflow divided by the duct’s cross-sectional area:
Velocity (FPM) = CFM ÷ Duct Area (ft²)
- Round duct area = π × (diameter ÷ 2)², with the diameter converted from inches to feet.
- Rectangular duct area = (width ÷ 12) × (height ÷ 12) ft².
For rectangular duct, the tool also computes the equivalent round diameter using the Huebscher equation, so you can carry the result into a round-duct friction chart:
De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25 (a, b in inches)
Worked example
1,200 CFM through a 14” round duct: area = π × (14/12/2)² = π × (0.583)² ≈ 1.069 ft², so velocity = 1,200 ÷ 1.069 ≈ 1,122 FPM — high for a trunk; a 16” duct would bring it down.
Target velocities (ACCA Manual D)
| Location | Typical FPM |
|---|---|
| Supply trunk | 600–900 |
| Supply branch | 400–600 |
| Return | 400–700 |
High velocity means noise and high static pressure; low velocity means poor distribution. Velocity is exact geometry — but full sizing also needs a friction rate and fitting losses from a Manual D friction chart or a ductulator, which depend on duct material and layout. Use this as a quick check, then size with Manual D, and confirm the system with the Static Pressure Calculator.
How FieldPad helps in the field
Check duct velocity here, then record your airflow findings in FieldPad against the equipment and the job. Turn an undersized-duct diagnosis into a client-ready estimate, and keep the measurements on the same record as the invoice — offline, on your iPhone.
Stop Re-Running These Numbers by Hand
FieldPad is the all-in-one iPhone CRM for solo HVAC techs — clients, scheduling, estimates, invoices with signatures, inventory, and equipment history. Keep your readings on the job, the client, and the equipment, all in one offline-ready app. EPA Section 608 and Subpart C compliance included.
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