HVAC Duct Size Calculator (CFM → Diameter)
Convert supply CFM and design velocity into duct area, round diameter, or rectangular size. Use this for hvac duct size calculator and trunk screening—not Manual D pressure design.
Input parameters
Quick: CFM + velocity. Advanced: circular vs rectangular and fix one side.
Quick examples:
About this calculator
Screens duct cross-section with the velocity method after CFM is known. For room turnover CFM use ACH to CFM; for tons/BTU use the HVAC size calculator. Browse all tools on the HVAC Calculators Hub.
Calculation results
What you are seeing: free-area size if air moved uniformly at your design velocity. Not modeled: fittings, friction chart, aspect-ratio limits, or Manual D. Next: HVAC size for tons, or Industrial HVAC Design for plant duct context.
Engineering disclaimer
Velocity-method estimates only. Final duct design, acoustic limits, and code compliance require detailed friction/fitting analysis and a qualified professional when required. Not a substitute for ACCA Manual D.
Design velocity bands (screening)
| Application | Typical velocity (ft/min) |
|---|---|
| Main trunk (industrial) | 2,000–2,500 |
| Branch / takeoff | 1,500–2,000 |
| Low-noise occupied | 700–1,200 |
| Exhaust / hood (generic) | 2,500–3,500 |
Formula & explanation
Free area A (ft²) = CFM ÷ V where V is design velocity in ft/min.
Round diameter D (in) = 12 × √(4A / π).
Rectangular: side₂ (in) = (A × 144) ÷ side₁.
Worked example: 12,000 CFM at 2,200 ft/min → A = 5.45 ft² → round D ≈ 31.6 in. Prefer standard sizes upward after acoustic and friction review.
Guides: Industrial HVAC Design — duct sizing · HVAC sizing · HVAC Calculators Hub
CFM vs round diameter (at 2,200 ft/min)
| CFM | Area (ft²) | Round Ø (in) |
|---|---|---|
| 1,000 | 0.45 | ~9.1 |
| 2,000 | 0.91 | ~12.9 |
| 5,000 | 2.27 | ~20.4 |
| 12,000 | 5.45 | ~31.6 |
Frequently asked questions
How do you calculate HVAC duct size from CFM?
Divide CFM by design velocity (ft/min) to get area in ft². For round duct, diameter in inches is 12 × √(4A/π). Enter values above for an instant result.
What velocity should I use for an HVAC trunk size calculator?
Industrial trunks often screen near 2,000–2,500 ft/min; branches 1,500–2,000. Lower velocity for quieter spaces. Always confirm owner and code criteria.
Is this Manual D duct design?
No—velocity-area screening only. Equal-friction Manual D, fittings, and static regain need detailed design.
How is this different from the HVAC size calculator?
The HVAC size calculator estimates tons/BTU/h. This page sizes duct after you know CFM—use ACH to CFM if you only have room volume and air changes.
How do I calculate duct size for HVAC step by step?
(1) Get CFM, (2) pick velocity, (3) compute area = CFM ÷ V, (4) convert to round or rectangular size, (5) bump to a standard catalog size and check noise/static in design software.