Duct Size Calculator
Size HVAC ductwork by CFM and velocity
Cubic feet per minute required
Standard residential: 600 fpm
Duct Area
—
sq in
Duct Diameter
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in
Recommended Shape
—
Disclaimer: Results are estimates based on standard formulas. Not a substitute for professional advice from a licensed contractor, engineer, or architect. Verify all calculations before making purchasing or construction decisions.
How to Use This Calculator
Correctly sized ductwork ensures balanced airflow, quiet operation, and efficient HVAC performance. Undersized ducts restrict flow and strain equipment, while oversized ducts waste material and money.
- Determine the CFM required for the room or branch
- Set your target air velocity (residential: 600 fpm)
- Enter both values to calculate the duct diameter
- Use the recommended shape to guide material selection
Expert Insights
Smart analysis for your HVAC project
Velocity Optimization
Higher velocity shrinks duct size but increases noise and friction. AI balances 400-900 fpm for residential comfort and quiet operation.
Shape Selection Logic
Round ducts under 8 inches are quieter and more efficient. Larger runs benefit from rectangular profiles to save headroom.
Friction Awareness
Every elbow and transition adds friction loss. AI encourages upsizing ducts slightly on long or complex runs to maintain airflow.
Calculation Formula
Duct Area (sq ft) = CFM / Velocity
Duct Area (sq in) = Duct Area (sq ft) x 144
Duct Diameter = ceil(sqrt(Area / pi) x 2)
Recommended Shape = Diameter < 8 in ? Round : Rectangular
Duct Material Comparison
| Material Type | Cost per ft | Airflow Efficiency | Noise Level | Best For |
|---|---|---|---|---|
| Galvanized Sheet Metal | $8-15 | Excellent | Low | Main trunks |
| Flexible Duct | $3-6 | Fair | Medium | Short branch runs |
| Fiberglass Duct Board | $5-10 | Good | Low | Quiet returns |
| Spiral Round Metal | $12-20 | Excellent | Very Low | Exposed ceilings |
Authority References
Our calculations are based on guidelines and standards from these authoritative sources:
- ACCA (Air Conditioning Contractors of America) — Manual D Residential Duct Systems. Learn more
- ASHRAE — Fundamentals Handbook — Duct Design. Learn more
- SMACNA — HVAC Duct Construction Standards. Learn more
Frequently Asked Questions
How do you calculate duct size from CFM?
Duct area (sq ft) = CFM / velocity (fpm). Convert to square inches (x144), then solve for diameter using the circle area formula. Standard residential velocity is 600 fpm.
What is the standard velocity for residential ducts?
Residential supply ducts typically run 600-900 fpm, while return ducts run 400-700 fpm. Lower velocities reduce noise but require larger ducts.
Round or rectangular ducts — which is better?
Round ducts are more efficient and quieter for small diameters (under 8 inches). Rectangular ducts save headroom in tight spaces and are used for larger runs.