Calculate the correct duct size for your HVAC system based on the required airflow in CFM. Enter the CFM and maximum air velocity to determine the appropriate round duct diameter or equivalent rectangular duct dimensions for efficient, quiet air distribution.
Enter Your Measurements
Results
Round Duct Diameter
11
inches
Exact Diameter
10.2
inches
Actual Air Velocity
606
FPM
Cross-Section Area
82.3
sq in
Rectangular Width
12
inches
Rectangular Height
7
inches
Formula
How to Use This Calculator
How to Use
- 1
Enter the required airflow in CFM (typically 1 CFM per sq ft of room area, or 400 CFM per ton of AC).
- 2
Enter the maximum air velocity in feet per minute (600-700 FPM for main ducts, 400-500 for branches).
- 3
Results show both round and equivalent rectangular duct sizes.
Frequently Asked Questions
Frequently Asked Questions
What size duct for 400 CFM?
How many CFM per ton of AC?
What is the maximum air velocity for residential ducts?
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About This Calculator
This calculator uses the formula: Area = CFM ÷ Velocity | Diameter = √(4 × Area ÷ π) | Velocity should not exceed limits for noise control. All calculations follow industry-standard methods. Results are estimates — always verify with a licensed professional for structural or code-compliant work.
Built and maintained by the CalcSmart team. Last updated March 2026.
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