How to use this calculator
- Enter free-air demand. Use the expected SCFM or Nl/min through this pipe run.
- Enter line pressure. Use gauge pressure at the start of the run.
- Describe the run. Enter actual pipe ID, pipe length, roughness and fitting K allowance.
- Set limits. Enter a velocity target and allowable pressure drop for the run.
- Choose actual pipe ID. Use the larger of the velocity-based and pressure-drop-based required IDs, then round up with current pipe data.
How it works
This compressed air pipe sizing calculator starts with free-air demand, such as SCFM or Nl/min. It converts that free-air flow into mass flow with the ideal gas law, then estimates actual line velocity at compressed-air density.
m_dot = P_atm * Q_free / (R * T)
v = (m_dot / rho_line) / A
Pressure drop is then screened with Darcy-Weisbach over the entered pipe length and fitting loss allowance. The density is iterated at the average pipe pressure so a larger calculated pressure drop feeds back into the air density estimate.
dp = (f * L / D + sum K) * rho * v^2 / 2
The required actual ID is the larger of the diameter needed to stay under the velocity target and the diameter needed to stay under the pressure drop allowance.
Worked example
Verified against the live calculator
A 35 SCFM shop-air branch at about 100 psi
through 100 ft of roughly 1 in actual ID pipe
is only a few psi of drop with the default fitting allowance. If the
pressure-drop utilization rises above the allowance, increase actual ID
or reduce equivalent length/fittings before increasing compressor
pressure.
Frequently asked questions
How do you size compressed air pipe from SCFM?
Convert free-air flow to actual line flow at the operating pressure, check velocity from Q/A, then check pressure drop over the pipe length and fittings. The required pipe ID is the larger ID needed by velocity and by allowable pressure drop.
Why is free-air flow different from line flow?
SCFM or Nl/min is free-air flow at atmospheric pressure. Inside a compressed-air line, the same mass flow occupies less volume because the air is denser at line pressure.
Does this choose nominal pipe size?
No. It reports required actual inside diameter. Choose the nearest larger nominal pipe size from current pipe, copper, tube, hose or airline manufacturer data.
Does this include compressor controls or dryers?
No. It is a distribution pressure-drop screen. Compressor controls, receiver storage, dryers, filters, regulators, branch diversity, leakage and manufacturer component data still control the final system.
Method & assumptions
- Free-air flow is treated as atmospheric-pressure air at the entered temperature.
- Pressure drop uses Darcy-Weisbach with Swamee-Jain turbulent friction and user-entered roughness.
- Fittings, filters, quick-connects, dryers and valves are represented only by the entered sum-K value.
- Outputs are required actual IDs, not nominal pipe schedules or manufacturer tube/hose sizes.
- Does not model compressor controls, receiver storage, branch diversity, leakage, dryer/filter pressure drop, regulators, condensate, noise limits, code requirements or manufacturer ratings.
Related compressed-air workflow
Pair this with pneumatic air consumption for demand, compressed-air receiver sizing for storage, compressed-air leak cost for wasted demand, and pneumatic valve Cv for component pressure drop.