Hydraulic Calculations Made Simple
Two numbers do most of the work in a hydraulic system. Pressure primarily determines available force — the harder the oil pushes on an area, the more force you get. Flow primarily determines actuator speed — the more oil you send per minute, the faster a cylinder fills and moves.
Real systems don't behave perfectly. Pressure drops through hoses and valves, back pressure on the return side, seal friction, internal leakage and relief valve settings all change what actually reaches the load. The calculator above gives theoretical values — a solid starting point, not a guarantee.
How hydraulic cylinder force is calculated
Force = pressure × area. When a cylinder extends, oil pushes on the whole piston face, so the area is π × bore² ÷ 4. With pressure in PSI and area in square inches, the answer is in pounds-force (lbf). In metric, bar × 0.1 gives N/mm², and N/mm² × mm² gives newtons.
Why retract force is lower on a single-rod cylinder
On the rod side, the rod itself covers the middle of the piston. Oil can only push on the ring around it — the annular area, equal to piston area minus rod area. Same pressure, smaller area, less force. A bigger rod means a bigger difference between push and pull.
How hydraulic flow affects cylinder speed
Speed = flow ÷ effective area. One US gallon is 231 cubic inches, so GPM × 231 ÷ 60 gives cubic inches per second. Divide that by the area being filled and you get inches per second. Double the flow and, in theory, you double the speed.
Why retract speed is usually higher than extend speed
With the same flow going to either end, the rod side has less volume to fill for every inch of travel because the rod occupies some of it. Less volume per inch means the piston moves faster on the way back.
How cylinder chamber volume is calculated
Volume = effective area × stroke. The cap end holds piston area × stroke; the rod end holds annular area × stroke. Divide cubic inches by 231 for US gallons. This oil moves in and out as the cylinder cycles — it returns to the reservoir rather than being consumed — but the reservoir and pump must be able to supply the difference between the two chambers.
What theoretical hydraulic horsepower means
Hydraulic horsepower (PSI × GPM ÷ 1,714) is the power carried by the moving, pressurised oil. It is not the size of the engine or electric motor you need. Pumps are not 100% efficient, and heat and other losses mean the prime mover must supply more than this figure.
Why actual machine performance may differ
Pressure losses in lines, fittings and valves, back pressure, seal friction, leakage past the piston, pump wear, oil temperature and viscosity, and relief settings can all reduce real-world force and speed. Always check against manufacturer data for the components you are using.