How to use this calculator
- Enter the bore and stroke. Enter the cylinder bore (piston diameter) and the stroke length.
- Enter the working pressure. Enter the supply pressure at the cylinder, gauge (above atmospheric).
- Enter the cycle rate. Enter the number of full extend + retract cycles per minute.
- Read the consumption. Read the free air per cycle and the free-air consumption rate in Nl/min (or SCFM), plus the compression ratio.
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
Verified against the live calculator
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.