Ductwork Sizing Calculator

Size ductwork based on CFM requirements and velocity

Project Details
Enter your measurements and requirements
Results
Calculated estimates for your project
Recommended Duct Size8" diameter (round)
Actual Air Velocity573 FPM
Duct Area50.3 sq in
Noise LevelQuiet
Friction Loss (per 100 ft)0.057" w.c.
RecommendationSize is appropriate

Additional Information

About This Calculator

Our ductwork sizing calculator determines the proper duct dimensions based on required airflow (CFM) and maximum velocity. Properly sized ducts deliver conditioned air efficiently without excessive noise, while undersized ducts create airflow problems and increase energy costs.

The calculator offers both round and rectangular duct options and matches your CFM requirements to standard duct sizes. It calculates the actual air velocity and provides noise level guidance - residential systems should keep velocity below 900 FPM for quiet operation.

This tool is valuable for HVAC installers, homeowners adding runs to existing systems, and anyone asking "what size duct do I need?" or "how do I size HVAC ducts?" Proper duct sizing ensures your HVAC system performs as designed and maintains comfort throughout your home.

How to Use This Calculator

Follow these steps to size your ductwork:

  1. Determine the CFM requirement for the room or run
  2. Enter maximum desired air velocity (900 FPM typical for residential)
  3. Select round or rectangular duct type
  4. Review the recommended duct size and actual velocity
  5. Check noise level guidance and adjust if needed
Frequently Asked Questions

How do I calculate CFM for a room?

General rule: multiply room square footage by ceiling height, then divide by 60 for air changes per hour. Or use 1 CFM per square foot for typical rooms, 1.5 CFM for kitchens/bathrooms. A 200 sq ft bedroom needs approximately 200 CFM. HVAC load calculations provide more accurate values.

Round vs rectangular ducts - which is better?

Round ducts have less friction, are easier to seal, and move more air per material used. Rectangular ducts fit in tight spaces (between joists) and are easier to fabricate. Spiral round duct is the gold standard; rectangular flex duct has the most friction.

Why is my HVAC system noisy?

Noise usually indicates high air velocity from undersized ducts, too many turns, or dampers throttled too far. Velocity over 900 FPM creates noticeable noise; over 1200 FPM is very noisy. Solutions include larger ducts, proper transitions, and turning vanes in elbows.

How does duct length affect sizing?

Longer runs have more friction loss, requiring larger ducts to maintain airflow. Every 100 feet of duct adds friction. Fittings (elbows, tees) add "equivalent length" - a 90-degree elbow equals 10-15 feet of straight duct. Keep runs short and minimize turns.

Can I reduce duct size to fit in tight spaces?

Reducing size increases velocity and noise while decreasing airflow. If space is limited, switch to rectangular duct with the same cross-sectional area. A 6" round duct (28 sq in) equals approximately 8"x4" rectangular. Never reduce arbitrarily - maintain required area.

What is static pressure and why does it matter?

Static pressure measures resistance to airflow in the duct system, like water pressure in pipes. High static pressure (over 0.5" w.c.) indicates undersized ducts, dirty filters, or too many fittings. It forces the blower to work harder, reducing efficiency and increasing noise.

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