MPH to CFM Calculator: Convert Wind Speed to Airflow
Convert miles per hour (MPH) wind speed into Cubic Feet per Minute (CFM) using duct or vent dimensions. Useful in fan sizing, HVAC duct analysis, and wind tunnel design.
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How to Convert Wind Speed (MPH) to Airflow (CFM)
In HVAC, fluid dynamics, and ventilation engineering, it's often necessary to determine how much air is flowing through a duct or opening given the wind speed. CFM (Cubic Feet per Minute) is a volumetric flow rate, while MPH (Miles per Hour) is a linear speed. To convert between the two, we need to consider the cross-sectional area of the duct or vent through which air moves.
Formula for MPH to CFM
To calculate airflow (CFM) from wind speed (MPH), use the formula:
CFM = Area (ft²) × Velocity (ft/min)
Steps:
- Convert duct width and height from inches to feet
- Calculate area in ft²
- Convert wind speed from MPH to feet per minute (MPH × 88)
- Multiply area × velocity to get CFM
Example
Given:
- Wind Speed: 10 MPH
- Duct Size: 12" × 12"
Convert to feet: 1 ft × 1 ft = 1 ft²
Velocity = 10 × 88 = 880 ft/min
CFM = 1 × 880 = 880 CFM
Applications of MPH to CFM Conversion
- HVAC Sizing: Design supply and return ducts to maintain airflow based on expected fan speed or ambient flow.
- Exhaust Fan Performance: Evaluate airflow in ventilation systems based on measured speed at the duct exit.
- Wind Tunnel Testing: Convert test section speed to actual airflow rates.
- Environmental Control: Determine if wind-driven systems deliver sufficient air to spaces.
Units Used in This Calculator
- MPH (Miles per Hour): Measures wind or air speed
- CFM (Cubic Feet per Minute): Measures volume of air moved per minute
- Inches (Width × Height): Duct or opening size in square inches
Conversion Factors
- 1 MPH = 88 feet per minute (fpm)
- 1 inch = 0.08333 feet
- Area (ft²) = (Width × Height) / 144
Frequently Asked Questions
Q: Can I use this calculator for round ducts?
A: This version assumes rectangular cross-sections. For round ducts, calculate area as A = π × (d/2)², then apply the same formula.
Q: Is this calculation valid for turbulent flow?
A: Yes, but it's based purely on velocity and area — it doesn't account for turbulence effects on pressure loss or efficiency.
Q: What if I don’t know duct size?
A: You cannot determine CFM from MPH alone — area is essential to convert linear speed to volume.
Q: How accurate is MPH to CFM conversion?
A: It’s highly accurate when duct dimensions and wind speed are known. For real systems, consider also static pressure, temperature, and turbulence.
Tips for Accurate CFM Calculation
- Use anemometers to get precise wind speed at the duct opening
- Ensure duct dimensions are measured at the same point as speed
- Apply correction factors if air is highly humid, hot, or cold
- Consider averaging multiple readings in unstable flows
Conclusion
Converting wind speed (MPH) to airflow (CFM) is essential in many HVAC and fluid dynamics contexts. With this calculator, you can easily perform the conversion by inputting wind velocity and duct dimensions. Whether you're sizing fans, validating system airflow, or designing ventilation strategies, this tool saves time and improves accuracy.