CFM to FPS Calculator

Understanding CFM to FPS Conversion

Converting CFM (Cubic Feet per Minute) to FPS (Feet per Second) is essential for HVAC design, ventilation systems, and ductwork calculations. This conversion helps determine the velocity of air moving through a duct or pipe.

The Formula

The conversion formula is:

[
\text{FPS} = \frac{\text{CFM}}{\text{Cross-Sectional Area}} \times \frac{1}{60}
]

Where:

• FPS = Velocity in feet per second
• CFM = Air volume flow rate in cubic feet per minute
• Cross-Sectional Area = The area through which air flows (in ft²)
• 1/60 = Conversion factor from minutes to seconds

Step-by-Step Calculation

1. Measure the CFM: Determine the total airflow rate in cubic feet per minute
2. Calculate the Area: Find the cross-sectional area of the duct or pipe (π × r² for circular, length × width for rectangular)
3. Divide CFM by Area: This gives you velocity in feet per minute
4. Convert to Seconds: Divide by 60 to get feet per second

Example Calculation

Given:

• Total CFM = 300 cubic feet per minute
• Cross-Sectional Area = 10 ft²

Calculation:
[
\text{FPS} = \frac{300}{10} \times \frac{1}{60} = 30 \times \frac{1}{60} = 0.5 \text{ ft/s}
]

Result: 0.5 ft/s

Practical Applications

HVAC System Design

• Sizing supply and return ducts
• Ensuring proper air distribution
• Meeting building codes and standards
• Optimizing energy efficiency

Ventilation Engineering

• Calculating exhaust fan requirements
• Designing laboratory fume hoods
• Industrial ventilation systems
• Clean room airflow management

Duct Sizing Guidelines

• Main trunks: 1.0-2.5 ft/s
• Branch ducts: 0.5-1.5 ft/s
• Return air: 0.3-1.0 ft/s
• High-velocity systems: 2.5-4.0 ft/s

Common Cross-Sectional Areas

Circular Ducts:

• 6" diameter = 0.196 ft²
• 8" diameter = 0.349 ft²
• 10" diameter = 0.545 ft²
• 12" diameter = 0.785 ft²

Rectangular Ducts:

• 6" × 8" = 0.333 ft²
• 8" × 10" = 0.556 ft²
• 10" × 12" = 0.833 ft²
• 12" × 16" = 1.333 ft²

Tips for Accurate Calculations

1. Measure accurately: Use precise measurements for duct dimensions
2. Consider shape: Calculate area correctly based on duct shape (circular, rectangular, oval)
3. Account for fittings: Elbows and transitions affect actual velocity
4. Check standards: Follow ASHRAE guidelines for recommended velocities
5. Verify units: Ensure consistent use of feet and square feet throughout