How it works
A pneumatic cylinder is billed in free air — the volume drawn at
atmospheric pressure, which is what the compressor must supply. Start from the swept
volume of one stroke,
V = (π/4) · bore² · stroke
then scale it up by the compression ratio, gauge pressure referenced
to atmosphere:
CR = (P_gauge + P_atm) / P_atm
with P_atm = 1.013 bar. A full double stroke uses air on both the
extend and the retract, so the free air per cycle is 2 · V · CR. Multiply
by the cycles per minute to get the consumption rate. The same rod area that makes a
cylinder retract faster also displaces a little less air on the way back — this tool
uses the full bore both ways, so the figure is conservative.
Worked example
A 50 mm bore × 100 mm stroke cylinder at 6 bar (0.6 MPa) gauge, cycling 30 times a
minute. The compression ratio is (6 + 1.013) / 1.013 ≈ 6.9:1. Each stroke
sweeps ≈ 196 cm³, so a double stroke draws ≈ 2 × 196 × 6.9 ≈
2.7 free litres per cycle. At 30 cycles/min that is ≈
82 Nl/min (≈ 2.9 SCFM) of free-air consumption. The calculator returns
exactly this.
Frequently asked questions
- How do I calculate pneumatic cylinder air consumption?
- Find the swept volume V = (π/4)·bore²·stroke, multiply by the compression ratio (gauge + atmospheric)/atmospheric to get free air, double it for the extend + retract cycle, then multiply by the cycles per minute. Enter bore, stroke, pressure and cycle rate above and the calculator does it for you.
- What is free air (FAD) and how does it differ from the compressed volume?
- Free air delivery (FAD) is the volume measured at atmospheric pressure — the volume the compressor actually has to draw in. The cylinder’s compressed volume is much smaller; you scale it up by the compression ratio (gauge pressure + 1.013 bar, divided by 1.013 bar) to get free air. At 6 bar gauge that ratio is about 6.9:1.
- How do I size a compressor for my cylinders?
- Add up the free-air consumption (in Nl/min or SCFM) of every cylinder running at its real cycle rate, then add a margin — typically 25–30% — for valve and fitting leakage, future consumers and duty cycle. Match that total to the compressor’s FAD rating, not its displacement.
- Does the rod reduce the air consumption?
- Slightly. On the retract stroke the rod occupies part of the bore, so the rod side displaces a little less air than the full bore. This calculator uses the full bore area on both strokes, which over-states retract air slightly — a deliberately conservative estimate that also covers dead volume in the ports and lines.
- What is SCFM?
- Standard cubic feet per minute — the imperial measure of free-air consumption, the volume of air per minute referenced to standard atmospheric conditions. It is the imperial counterpart of Nl/min (1 SCFM ≈ 28.3 Nl/min). Toggle to imperial to read the result in SCFM.
- Does it work in metric and imperial?
- Yes — toggle SI/Imperial in the header. Bore and stroke switch between mm and inches, pressure between bar and psi, and consumption between Nl/min and SCFM.
Method & assumptions
- Uses the full bore area on both strokes — conservative; it ignores the rod displacement on retract and the dead volume in ports and lines.
- Atmospheric pressure taken as 1.013 bar (0.1013 MPa); pressure entered is gauge (above atmospheric).
- Isothermal free-air conversion (compression ratio only) — it does not model temperature change, line pressure drop or leakage. Add a margin (typically 25–30%) when sizing a compressor.
Related calculators
- Hydraulic Cylinder Force Calculator — Push and pull force with the rod-area differential shown explicitly. Standard ISO bores.
- Hydraulic Pump Flow & HP Calculator — Pump output flow and hydraulic/shaft power from displacement, speed and pressure.