CFM to MPH Calculator: Convert Airflow to Wind Speed
Convert airflow from Cubic Feet per Minute (CFM) into wind speed in Miles per Hour (MPH) based on duct or nozzle dimensions. Useful for engineers, HVAC specialists, and ventilation design professionals.
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Understanding CFM to MPH Conversion
CFM (Cubic Feet per Minute) is a volumetric measure of airflow — it tells you how much air moves. MPH (Miles per Hour) is a velocity unit — it tells you how fast air moves. To convert CFM to MPH, you need to know the cross-sectional area of the duct or nozzle through which the air is traveling. Without the area, you cannot calculate velocity from flow rate.
Conversion Formula
1. Convert duct size from inches to feet:
width_ft = width_in / 12
height_ft = height_in / 12
2. Compute area (A) in ft²:
A = width_ft × height_ft
3. Velocity in ft/min:
velocity_fpm = CFM / A
4. Convert to MPH:
MPH = velocity_fpm ÷ 88
Example
Given:
- CFM = 1200
- Duct = 24" × 18"
Area = (24×18)/144 = 3 ft²
Velocity = 1200 ÷ 3 = 400 ft/min
MPH = 400 ÷ 88 = 4.55 MPH
Why This Matters
- Fan Sizing: Ensures the fan produces sufficient airflow at target velocity
- Ventilation Design: Duct speed affects noise, static pressure, and efficiency
- Comfort & Safety: Prevents draftiness or inadequate ventilation
- Process Control: Many drying, cooling, or combustion processes rely on velocity
CFM vs. MPH - What's the Difference?
| Feature | CFM | MPH |
|---|---|---|
| What it Measures | Air volume per minute | Air velocity (distance per hour) |
| Dependent on Duct Size? | No | Yes |
| Typical Unit | Cubic feet/min | Miles/hour |
| Used For | Airflow rate | Air speed through a vent |
Common Ranges
- Home HVAC ducts: 2–5 MPH
- Workshop exhausts: 6–12 MPH
- Commercial blowers: 10–30 MPH
- Wind tunnels: 30–300+ MPH
Limitations
- This calculator assumes constant airflow — does not account for system pressure loss or turbulence
- For round ducts, you’ll need to calculate area as: A = π × (d ÷ 2)²
- MPH conversion uses 1 MPH = 88 feet per minute
Frequently Asked Questions
Q: Can I use this for round ducts?
A: Yes, just calculate the area in ft² manually for your duct and enter it in place of width × height.
Q: What if I only know CFM?
A: You must also know the duct size to convert to MPH — velocity is always area-dependent.
Q: Is this accurate for compressible flows?
A: This calculator assumes incompressible air — suitable for most HVAC and general-purpose airflow systems under 100 MPH.
Q: Can I reverse the calculation?
A: Yes — use the MPH to CFM calculator for the reverse operation.
Conclusion
Knowing the velocity of air based on flow rate helps with effective system design and diagnostics. This CFM to MPH calculator makes it easy to translate volumetric airflow into wind speed, especially when analyzing ductwork, fan outlets, or wind tunnels. Always verify duct dimensions, and use this calculator to get a clear picture of airflow velocity in your system.