Calculator
HVAC Duct Size Calculator
Size supply, return, and trunk duct runs from airflow (CFM) and target velocity — get round and rectangular dimensions, the next standard size up, and friction loss screening. Free, no signup.
Free
Sizing method
Bend counts
Number of elbows in the run. Each adds an equivalent straight length: 45° ≈ 7 ft, 90° ≈ 15 ft, 180° ≈ 30 ft (ACCA Manual D simplified).
45°
90°
180°
400 CFM · 700 FPM target
Recommended duct size
12" round
Calculated 10.2 in round equivalent
Actual velocity
509 FPMWithin typical residential range
Cross-sectional area
82 in²0.571 ft²
Est. friction loss
0.038 in.wg0.023 in.wg over 60 ft TEL
Select the next standard round size up from the calculated 10.2 in — recommended 12" round at 509 FPM.
What you'll get
Enter airflow (CFM)
Use room CFM from a load calculation, the 400 CFM/ton rule, or measured airflow at the register.
Choose application and duct type
Pick branch, trunk, or return presets for target velocity. Select round or rectangular and duct material.
Get duct dimensions
See recommended round size, rectangular options by aspect ratio, actual velocity at standard size, and friction loss per 100 ft.
How it works
- 1
Enter airflow (CFM)
Use room CFM from a load calculation, the 400 CFM/ton rule, or measured airflow at the register.
- 2
Choose application and duct type
Pick branch, trunk, or return presets for target velocity. Select round or rectangular and duct material.
- 3
Get duct dimensions
See recommended round size, rectangular options by aspect ratio, actual velocity at standard size, and friction loss per 100 ft.
Why proper duct sizing matters for HVAC performance
Undersized ducts create excessive air velocity, register noise, and high static pressure that starves the blower. Oversized ducts waste material and can reduce air throw in heating mode. This calculator uses the standard relationship Area = CFM ÷ Velocity to size round and rectangular ducts for residential and light commercial work.
For whole-system design, pair this with the HVAC Load Calculator and BTU Calculator to establish room CFM first, then size each run. For final Manual D duct design on complex systems, verify friction loss and fitting equivalent lengths with full ACCA/SMACNA software.
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View statesDuct Size Calculator FAQs
How do I calculate duct size from CFM?
Divide CFM by your target air velocity (FPM) to get required cross-sectional area in square feet. For round ducts, diameter = 12 × √(4 × area ÷ π). Always select the next standard duct size up from the calculated dimension.
What air velocity should I use for HVAC ducts?
Use 600–700 FPM for quiet bedroom branches, 700–800 FPM for living areas, 800–900 FPM for main trunks in unconditioned spaces, and 400–500 FPM for return air ducts. Higher velocity reduces duct size but increases noise.
What size duct do I need for 400 CFM?
At 700 FPM (typical branch), 400 CFM needs about 82 in² of area — roughly a 10-inch round duct. At 600 FPM for quieter bedrooms, upsize to about 11 inches (use 12-inch standard). Enter your CFM and velocity in the calculator for exact sizing.
Is this a full Manual D duct design?
No. This is a screening-level CFM-to-size calculator. Full Manual D accounts for fitting equivalent lengths, available static pressure, and system-wide balancing. Use this for field sizing and customer conversations; run Manual D software before final design on complex systems.
