Air Duct Size Calculator

Duct Area Formula:

\[ Area (in²) = \frac{CFM \times 144}{Velocity (fpm)} \]

Area (in²):

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1. What is the Air Duct Size Calculator?

The Air Duct Size Calculator determines the cross-sectional area required for ductwork based on airflow rate (CFM) and air velocity. Proper duct sizing is essential for efficient HVAC system performance and energy conservation.

2. How Does the Calculator Work?

The calculator uses the duct area formula:

\[ Area (in²) = \frac{CFM \times 144}{Velocity (fpm)} \]

Where:

\( CFM \) — Airflow rate in cubic feet per minute
\( Velocity \) — Air velocity in feet per minute
\( 144 \) — Conversion factor (12² for square inches)
\( Area \) — Duct cross-sectional area in square inches

Explanation: This formula calculates the required duct area to maintain proper airflow at the specified velocity, ensuring efficient system operation.

3. Importance of Duct Sizing

Details: Proper duct sizing is crucial for maintaining adequate airflow, minimizing pressure drops, reducing energy consumption, and preventing noise issues in HVAC systems.

4. Using the Calculator

Tips: Enter CFM (cubic feet per minute) and velocity (feet per minute). Both values must be positive numbers. Typical residential duct velocities range from 600-900 fpm for supply and 500-700 fpm for return air.

5. Frequently Asked Questions (FAQ)

Q1: What is the recommended velocity for residential ductwork?
A: For supply ducts: 600-900 fpm; for return ducts: 500-700 fpm. Lower velocities reduce noise but require larger ducts.

Q2: How do I convert square inches to round duct diameter?
A: Use the formula: Diameter = 2 × √(Area/π). For rectangular ducts, common aspect ratios are used to determine dimensions.

Q3: What happens if ducts are undersized?
A: Undersized ducts cause increased pressure drop, reduced airflow, higher energy consumption, noise problems, and potential system damage.

Q4: Are there standard duct sizes?
A: Yes, common round duct sizes include 4", 5", 6", 7", 8", 10", 12", 14", etc. Rectangular ducts follow standard dimension combinations.

Q5: How does altitude affect duct sizing?
A: At higher altitudes, air is less dense, so CFM requirements may need adjustment. Consult HVAC manuals for altitude correction factors.