Rectangular Duct CFM Calculator HVAC

CFM Formula for Rectangular Duct:

\[ CFM = Velocity \times \frac{Length \times Width}{144} \]

CFM (Cubic Feet per Minute):

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is the Rectangular Duct CFM Formula?

The CFM (Cubic Feet per Minute) formula for rectangular ducts calculates the airflow volume through a rectangular duct based on air velocity and duct dimensions. It's essential for proper HVAC system design and airflow management.

2. How Does the Calculator Work?

The calculator uses the rectangular duct CFM formula:

\[ CFM = Velocity \times \frac{Length \times Width}{144} \]

Where:

\( Velocity \) — Air velocity in feet per minute (ft/min)
\( Length \) — Duct length in inches (in)
\( Width \) — Duct width in inches (in)
\( 144 \) — Conversion factor (square inches to square feet)

Explanation: The formula calculates the cross-sectional area of the duct in square feet and multiplies it by the air velocity to determine the volume of air flowing through the duct per minute.

3. Importance of CFM Calculation in HVAC

Details: Accurate CFM calculation is crucial for proper HVAC system design, ensuring adequate airflow for heating and cooling, maintaining indoor air quality, and optimizing energy efficiency in ventilation systems.

4. Using the Calculator

Tips: Enter air velocity in ft/min, duct length and width in inches. All values must be valid positive numbers. The calculator will compute the CFM value for rectangular duct systems.

5. Frequently Asked Questions (FAQ)

Q1: Why is the 144 conversion factor used?
A: The factor 144 converts square inches to square feet (since 1 square foot = 144 square inches), ensuring consistent units throughout the calculation.

Q2: What are typical velocity ranges for HVAC ducts?
A: Typical velocities range from 600-900 ft/min for low-pressure systems and 1200-1800 ft/min for high-pressure systems, depending on application and noise considerations.

Q3: How does duct shape affect CFM calculation?
A: Different duct shapes (round, rectangular, oval) require different area calculation methods. This calculator is specifically designed for rectangular ducts.

Q4: Can this calculator be used for round ducts?
A: No, round ducts use a different formula: CFM = Velocity × π × (Diameter/2)² / 144, where the area is calculated using the circle area formula.

Q5: Why is CFM important in HVAC system design?
A: CFM determines the volume of air being moved through the system, which affects temperature control, humidity regulation, air filtration efficiency, and overall system performance.