CFM Calculator (Air Flow)
Calculate air flow in CFM from duct size and air velocity. Enter the duct geometry and the average velocity and the calculator returns the volumetric flow in CFM, m³/h and L/s for circular and rectangular ducts. Already have a figure to convert? Use the CFM to m³/h and m³/h to CFM converters.
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Schedule 40 pipe sizes, water flow in GPM and LPM at four design velocities, and recommended velocities by application. One PDF, made to pin above the bench.
How It Works
Air volumetric flow is calculated by multiplying average air velocity by duct cross-sectional area. For circular ducts, area is πD²/4. For rectangular ducts, area is width × height. Units are converted automatically.
Air Flow Equation
Q = V × A
Where:
- • Q = Volumetric flow rate (m³/s, CFM, or L/s)
- • V = Average air velocity (m/s or ft/min)
- • A = Duct cross-sectional area (m² or ft²)
Circular Duct Area
A = π × D² / 4
Where D is the internal duct diameter. For example, a 300 mm diameter circular duct has a cross-sectional area of 0.0707 m².
Rectangular Duct Area
A = W × H
Where W is duct width and H is duct height. A 400 mm × 200 mm rectangular duct has an area of 0.08 m².
Equivalent Circular Diameter
When comparing rectangular and circular ducts, the equivalent diameter for equal friction loss is:
D_eq = 1.30 × (W × H)^0.625 / (W + H)^0.250
Typical HVAC Velocity Ranges
| Application | m/s | FPM |
|---|---|---|
| Residential supply ducts | 2–4 | 400–800 |
| Commercial supply ducts | 5–8 | 1000–1600 |
| Main supply trunks | 6–10 | 1200–2000 |
| Return air ducts | 3–5 | 600–1000 |
| Exhaust ducts | 5–10 | 1000–2000 |
| Industrial ventilation | 10–20 | 2000–4000 |
Higher velocities reduce duct size but increase noise and pressure drop. ASHRAE guidelines recommend keeping supply velocities below 7.5 m/s (1500 FPM) in occupied spaces to control noise.
FAQ
Here you will find the answers to the frequently asked questions about air flow calculations.
Frequently Asked Questions
How do I convert between CFM and m³/h?
1 CFM (cubic feet per minute) equals 1.699 m³/h. Conversely, 1 m³/h equals 0.5886 CFM. For L/s: 1 CFM = 0.4719 L/s. This calculator automatically converts between all three units so you can work in whichever system your project requires.
What velocities are typical in HVAC ducts?
Residential supply ducts typically run at 2–4 m/s (400–800 FPM), commercial supply ducts at 5–8 m/s (1000–1600 FPM), and industrial ventilation at 10–20 m/s (2000–4000 FPM). Velocities above 7.5 m/s in occupied spaces may cause unacceptable noise levels per ASHRAE guidelines.
Should I use circular or rectangular ducts?
Circular ducts have lower friction losses per unit area and require less material for the same airflow capacity. However, rectangular ducts fit more easily in tight ceiling spaces and can be customized to available clearances. For equal friction performance, use the equivalent circular diameter formula to compare sizes.
How does air temperature affect flow calculations?
Air density changes with temperature. At sea level, air density is approximately 1.225 kg/m³ at 15°C but drops to about 1.127 kg/m³ at 40°C. While volumetric flow (CFM, m³/h) stays constant regardless of temperature, the mass flow rate changes. For heating/cooling load calculations, you may need to account for density differences between supply and return air.
What is the relationship between air flow and pressure drop in ducts?
Pressure drop in ducts increases approximately with the square of velocity. Doubling the air velocity roughly quadruples the pressure drop. This is why HVAC engineers balance duct sizing against fan energy costs — smaller ducts save material but require more fan power to overcome higher pressure drops. Use our Pressure Drop Calculator for detailed analysis.
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