Formula used
Continuity: Q = A × V. Circular area is A = πD² ÷ 4. Mass flow is ṁ = ρQ.
Pressure loss: ΔP = (fL ÷ D + ΣK) × ρV² ÷ 2. Reynolds number is Re = ρVD ÷ μ. Friction uses Colebrook–White, Swamee–Jain, or Haaland.
Choked-flow screening: the calculator checks the critical pressure ratio. It also estimates a nozzle-style upper mass-flow bound. Detailed long-pipe gas design may require Fanno or isothermal modelling.
How to use this calculator
- Select the calculation mode and intended application.
- Enter actual internal dimensions, length, and material roughness.
- Provide operating temperature, absolute pressures, and humidity.
- Set velocity, pressure-loss, Mach, and design-flow limits.
- Add fitting counts, custom losses, and design allowances.
- Calculate, review warnings, compare sizes, then export results.
Example data
| Application | Diameter | Length | Velocity limit | Pressure-drop limit |
|---|---|---|---|---|
| HVAC supply | 200 mm | 20 m | 8 m/s | 500 Pa |
| Compressed air | 50 mm | 40 m | 20 m/s | 25 kPa |
| Dust extraction | 250 mm | 30 m | 18 m/s | 1,200 Pa |
| Industrial blower | 400 mm | 75 m | 22 m/s | 3,000 Pa |
Frequently asked questions
What determines maximum airflow through a pipe?
Pipe area, velocity, friction, pressure, temperature, fittings, and gas compressibility determine capacity. The lowest permitted limit controls the result.
Why must internal diameter be used?
Nominal size does not equal open flow diameter. Schedule and wall thickness change area significantly.
When is airflow compressible?
Compressibility becomes important near Mach 0.30 or with large pressure changes. Detailed gas-flow methods are then preferable.
What is choked airflow?
Choked flow occurs when local gas velocity reaches sonic conditions. Lower downstream pressure then gives little extra mass flow.
Does this replace fan selection software?
No. It estimates system resistance and capacity. Final fan selection should use the manufacturer performance curve.
How are fittings included?
Each fitting adds a minor-loss coefficient. The calculator combines preset and custom K values.
What is the friction factor?
It is the Darcy factor used in pressure-loss calculations. Roughness and Reynolds number determine its value.
What does SCFM mean?
SCFM describes airflow at selected standard conditions. ACFM describes volume at actual operating conditions.
Why is pressure reference important?
Gas equations require absolute pressure. Gauge values must include local atmospheric pressure before calculations.
Can rectangular ducts be calculated?
Yes. The calculator uses actual area and hydraulic diameter. This provides an engineering approximation for noncircular passages.
Is the result suitable for safety-critical design?
Use it for preliminary sizing and checking. Safety-critical systems require applicable codes, verified data, and professional review.
Engineering note
This calculator provides preliminary engineering estimates. Real systems may involve heat transfer, leakage, pulsation, network effects, and changing gas properties. Confirm final designs with recognised standards and qualified engineering review.