The pneumatic cylinder force calculator above covers the compressible case: an air cylinder, where the working fluid changes volume with pressure and the supply pressure at the port falls as flow rises. That is a materially different problem from a hydraulic actuator, and this page is deliberately separate from the hydraulic cylinder force calculator, which handles the incompressible case and derives rod speed directly from pump flow. With oil, flow in equals volume swept and rod speed follows arithmetically. With air it does not, and that difference runs through everything below.
Compressed air also stores energy. A charged cylinder or a pressurised line holds real stored energy that is released the moment a fitting fails or a hose whips, and a trapped cylinder can move unexpectedly when a valve is operated or when residual pressure bleeds unevenly. Guarding, energy isolation and lockout on any machine using this actuator are the machine builder's and the employer's obligation under the applicable safety regulations, not something a calculator addresses.
What This Pneumatic Cylinder Force Calculator Does
Force is pressure multiplied by area, and for a cylinder that is straightforward: the extend stroke acts on the full bore area, while the retract stroke acts on the bore area minus the rod area. That annular difference is why an air cylinder always pulls with less force than it pushes, and why a machine designed around the push force can stall on the return.
The second half of the page is the part a hydraulic calculator has no equivalent for. Air is compressed at the cylinder and released to atmosphere at the exhaust, so the volume a cylinder swept at working pressure corresponds to a much larger volume of free air at atmospheric pressure. The ratio between them is the compression ratio, and multiplying by it converts swept volume into the free air the compressor actually has to supply. This is the number that sizes a compressor, and getting it wrong by leaving the ratio out understates demand by a factor of six or more.
The headline figure is the extend force at the pressure that reaches the cylinder, after friction losses. The grid gives the retract force, the free air one full cycle consumes, the free air demand at your cycle rate in both normal litres per minute and standard cubic feet per minute, and the compression ratio itself.
How to Use It
- Enter the bore, not the tube outside diameter. Bore is the internal diameter the piston seals against. A 63 mm bore cylinder has a tube noticeably larger than 63 mm on the outside.
- Estimate the pressure drop honestly. A regulator set to 6 bar does not deliver 6 bar at the piston while air is flowing. Fifteen per cent is a reasonable starting assumption for a typical installation, and a long run of small tubing through several fittings can be worse.
- Check the retract force against your load. On a cylinder with a large rod the retract area can be a third smaller than the bore area, which is a third less force on the return stroke.
- Use the free air demand to size supply, not the swept volume. The swept volume is the air at working pressure. The compressor has to deliver that multiplied by the compression ratio.
- Do not use this to set rod speed. Air is compressible, so speed depends on valve flow coefficient, tubing restriction, exhaust path, load inertia and the cylinder's own cushioning. It is not a division of flow by area.
The Formulas and a Worked Example
Bore area is Ab = πD²/4 and the annulus area is Aa = π(D² − d²)/4 for bore D and rod d. Extend force is F = p Ab η and retract force is F = p Aa η, with p the gauge pressure at the cylinder port and η the mechanical efficiency. Unit definitions follow NIST's SI Units reference, where one bar is exactly 100 kPa and one pound per square inch is 6,894.757 Pa.
Work the defaults through. A 63 mm bore gives π × 63² ÷ 4 = 3,117.2 mm². A 20 mm rod removes π × 20² ÷ 4 = 314.2 mm², leaving 2,803.1 mm² on the annulus side. A 6 bar supply losing 15 % arrives at 5.1 bar, which is 0.51 N/mm². Extend force is therefore 3,117.2 × 0.51 = 1,589.8 N theoretical, and at 90 % efficiency 1,430.8 N. Retract is 2,803.1 × 0.51 × 0.9 = 1,286.6 N, about 10 % less.
Now the air. Over a 100 mm stroke the extend side sweeps 3,117.2 × 100 = 311,725 mm³ = 0.3117 litres, and the retract side sweeps 0.2803 litres, giving 0.5920 litres per full cycle at working pressure. The compression ratio is absolute working pressure divided by atmospheric, (5.1 + 1.013) ÷ 1.013 = 6.03. So the free air per cycle is 0.5920 × 6.03 = 3.57 normal litres. At 30 cycles per minute that is 107 normal litres per minute, or 3.79 standard cubic feet per minute.
Notice the size of that correction. Anyone sizing a compressor from the 0.59 litre swept volume alone would specify roughly one sixth of the air the machine needs. This term simply does not exist in a hydraulic calculation, because oil is not compressed on the way in and not expanded on the way out.
Why Pneumatic Rod Speed Is Not the Hydraulic Calculation
In a hydraulic circuit, rod speed is pump flow divided by the effective area, because the oil is essentially incompressible and every cubic centimetre the pump delivers displaces the piston by a fixed distance. That calculation is exact enough to design with, and the live hydraulic page reports it for exactly that reason.
Air behaves nothing like this. The cylinder must first fill with compressed air before the piston moves significantly, and the fill rate depends on the pressure difference across the valve, which changes throughout the stroke. The exhaust side must simultaneously empty against atmosphere, and on many circuits the exhaust restriction rather than the inlet governs the speed — which is precisely why meter-out flow control is the standard way to regulate a pneumatic cylinder while meter-in is standard in hydraulics.
On top of that, a compressed air column is a spring. A pneumatic cylinder driving an inertial load can accelerate, overshoot, bounce and stick, and its motion depends on the load, the tubing volume, the valve's flow coefficient, the exhaust path and the end cushioning. There is no single division that yields rod speed, which is why this page reports force and air consumption and deliberately does not report speed. Real speeds come from the manufacturer's flow data for the specific valve and cylinder combination, or from measurement on the machine.
Where the Pressure Actually Goes
The gap between catalogue force and real force is usually pressure drop, and it is worth understanding where it accumulates. A filter-regulator-lubricator unit, a directional valve, several push-fit fittings and a few metres of small-bore tubing each take a share, and every one of those shares grows with flow rate. A cylinder that is barely moving sees almost the full supply pressure; the same cylinder moving fast may see substantially less.
That flow dependence produces a counterintuitive behaviour: an air cylinder is strongest when it is stalled and weakest when it is moving quickly, which is the opposite of the intuition most people bring from electric actuators. It also means a circuit that works fine when tested slowly can stall under a real duty cycle. Undersized tubing is the most common single cause, followed by a valve chosen on port thread size rather than on flow coefficient.
Two further effects reduce force. Back pressure on the exhaust side subtracts directly from the net force, so a restricted exhaust or a silencer clogged with oil mist costs push force. And seal friction rises when the cylinder has been standing, when the air is dry and unlubricated, or when the rod is side-loaded by misalignment. The efficiency box on this page is a blunt allowance for all of that; a cylinder with a genuinely sticky seal can be far worse than any percentage figure suggests.
Stored Energy, Guarding and Isolation
A pneumatic cylinder is not a low-energy device. The air in a charged cylinder and its connected lines holds stored energy that is released instantly if a fitting fails, and a whipping hose or an ejected fitting is a genuine hazard. Unlike a hydraulic circuit, where a leak produces a jet and a mess, a pneumatic failure produces a rapid expansion that can move parts violently.
A trapped cylinder is the other hazard worth naming. Air held in a cylinder with the supply removed will still drive the rod when a valve is operated, when residual pressure bleeds unevenly from the two sides, or when a control system is reset. Machinery that can move under stored energy has to be isolated and that energy released before anyone works on it, which is the entire subject of energy control and lockout procedure — in the United States, the OSHA standard on the control of hazardous energy (lockout/tagout), 29 CFR 1910.147, with equivalent requirements in other jurisdictions.
Guarding, energy isolation, dump valves, soft-start circuits and the risk assessment behind them are the responsibility of the machine builder and the employer operating the machine. This page computes force and air consumption. It makes no assessment of whether a given actuator, circuit or installation is safe, and no figure it produces should be read as authorising any arrangement.
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Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Using regulator pressure as cylinder pressure — the pressure that makes force is the one at the port while air is flowing, and the drop between them grows with speed.
- Sizing a compressor from swept volume — free air demand is the swept volume multiplied by the compression ratio, which at 6 bar is a factor of about six.
- Designing around the extend force only — the rod removes area on the retract side, so the pull is always weaker than the push, sometimes by a third or more.
- Calculating rod speed as flow divided by area — that is the incompressible result. Air fills and exhausts against changing pressure differences and behaves as a spring against an inertial load.
- Treating a de-pressurised supply as a safe machine — air trapped in the cylinder and lines is stored energy, and isolation and lockout procedures exist precisely because it can still move the rod.
Related Free Tools From Arb Digital
For the incompressible case, where flow in genuinely does give rod speed, use the hydraulic cylinder force calculator. Air consumption at standard conditions is handled in more detail by the SCFM calculator, and pressure units by the pressure converter. The gas behaviour underneath the compression ratio comes from the Boyle's law calculator and the ideal gas law calculator, while the air density calculator covers the altitude and temperature dependence of free air. For the distribution system, the pipe flow calculator covers the tubing that causes the pressure drop, and on the mechanical side the force calculator and the horsepower calculator handle the load and the compressor duty. Everything is on the free online tools hub.
Frequently Asked Questions
Hydraulic fluid is essentially incompressible, so rod speed follows directly from pump flow divided by area and that page reports it. Air is compressible, so this page adds a free air consumption term that has no hydraulic equivalent and deliberately does not report rod speed, because for air that depends on valve flow coefficient, tubing, exhaust path, load inertia and cushioning rather than on a division.
Mostly because of pressure drop between the regulator and the cylinder port. Filters, valves, fittings and small-bore tubing all take a share, and each share grows with flow, so a fast-moving cylinder sees less pressure than a stalled one. Seal friction, rod side loading and back pressure on the exhaust side all subtract further.
Because the rod occupies part of the piston area on the retract side. The extend stroke acts on the full bore area while the retract stroke acts on the bore area minus the rod area, so the pull force is lower in exactly that proportion. On a cylinder with a large rod the difference can exceed thirty per cent.
It is the volume of air at atmospheric pressure that the compressor must deliver to fill the cylinder at working pressure. It equals the swept volume multiplied by the compression ratio, which is absolute working pressure divided by atmospheric pressure. At 6 bar gauge that factor is about six, so sizing a compressor from swept volume alone understates demand badly.
No, and it deliberately does not report a speed. Air must fill the cylinder against a changing pressure difference while the other side exhausts to atmosphere, and on many circuits the exhaust restriction rather than the inlet governs the motion. Speed comes from manufacturer flow data for the specific valve and cylinder, or from measurement on the machine.
Because the compressed air behind the piston acts as a spring, so restricting the inlet gives an unstable, lurching motion against an inertial load. Restricting the exhaust instead keeps back pressure on the opposite side and gives controlled, repeatable movement. In hydraulics, where the fluid is incompressible, meter-in works fine.
No. A charged cylinder and its connected lines hold real stored energy, released instantly if a fitting or hose fails, and a cylinder with trapped air can still drive the rod after the supply is removed. Machinery that can move under stored energy must be isolated and that energy released before work begins, under lockout and energy control procedures that are the employer's and machine builder's responsibility.
This tool is provided for educational and preliminary engineering use. It computes ideal force and air consumption and makes no assessment of the safety, guarding or suitability of any actuator, circuit or machine. Compressed air stores energy that can cause injury. Machine guarding, energy isolation and lockout are the obligation of the machine builder and the employer under the applicable safety regulations, and component ratings are set by the manufacturer.
