Every hydraulic cylinder comes down to one honest equation — force equals pressure times area — and a handful of sanity checks around it. Get the bore, rod, and flow right and the cylinder does its job quietly for years. Get them wrong and you either stall under load or buckle a rod. Here is the whole sizing sequence, worked with real numbers.
Step 1 — Bore diameter from force and pressure
On extend (push), the full piston face sees system pressure:
Say the machine needs 50 kN of push force and the power unit runs at 160 bar (16 N/mm²). The required piston area is:
D = √(4A / π) = √(4 × 3125 / π) ≈ 63.1 mm
63.1 mm is not a stock bore, so round to the standard 63 mm and re-check what it actually delivers:
| Bore | Piston area | Push force @ 160 bar |
|---|---|---|
| 63 mm | 3,117 mm² | 49.9 kN |
| 80 mm | 5,027 mm² | 80.4 kN |
The 63 mm bore lands at 49.9 kN — a hair under target. If the 50 kN is a hard minimum, either accept a slightly higher relief setting or step up to 80 mm and gain a comfortable margin. That trade — pressure versus bore — is the first real decision in every hydraulic design.
Step 2 — Rod diameter and retract force
On retract (pull), the rod blocks part of the piston, so only the annulus area is pressurized:
A 63 mm bore typically pairs with a 36 mm rod. The rod area is 1,018 mm², so:
F_retract = 2,099 mm² × 16 N/mm² ≈ 33.6 kN
Retract force is only about two-thirds of push force — normal, and usually fine because most machines push hard and pull light. If your load reverses that (a puller, a press ejector), you may need to size the bore on the retract requirement instead.
Buckling — the check people skip
A long, thin rod under compression is a column, and columns buckle. Euler’s critical load depends on rod diameter, stroke, and how the cylinder is mounted (the end-fixity condition). A short, pinned-pinned cylinder tolerates a slender rod; a long cantilever mount does not. If your stroke is long relative to the rod diameter, verify the rod against Euler buckling before you commit — the same column theory in our Column Buckling calculator applies directly to the piston rod.
Step 3 — Speed from flow rate
Cylinder velocity is just flow divided by the area being filled:
Feed the 63 mm bore with 20 L/min (333,333 mm³/s):
Because retract fills the smaller annulus, the rod comes back faster for the same flow — about 159 mm/s here. If a cycle-time target drives the design, work this step backward: pick the velocity you need, then size the pump flow to match.
Step 4 — The rest of the circuit
A correctly sized cylinder starved by an undersized hose or throttled by a fitting still underperforms. Two quick checks close the loop:
- Pressure drop — velocity through the lines should stay in the accepted range (roughly 3–5 m/s pressure, under ~1.5 m/s suction). The Hose Pressure Drop calculator flags a line that will overheat the oil or waste pump power.
- Hose selection — match the hose ID and pressure class to the flow and 160 bar working pressure with the Hydraulic Hose Selector; the Thread Identifier sorts out whether those ports are JIC, ORB, BSPP, or NPT before you order fittings.
- Fitting torque — over- or under-torqued fittings are the classic leak source; the Fitting Torque calculator gives the right value by thread standard and size.
Step 5 — Seals decide whether it lasts
The cylinder can be perfectly sized and still weep past a mismatched seal. Piston and rod seals must suit the fluid, pressure, and temperature; the gland has to be cut to spec. Use the Hydraulic Seal Selector to choose the profile and material, then the Seal Groove calculator to dimension the gland. A quality seal kit from a name like Parker or SKF is cheap insurance against a rebuild.
Pressure intensification on the rod side
One subtlety catches even experienced designers: when a cylinder retracts against a blocked or restricted rod-side return, pressure on the annulus can intensify above supply pressure because the piston-side area pushing it is larger than the rod-side area receiving it. The ratio is simply the area ratio, A_piston / A_annulus. For our 63 mm bore / 36 mm rod that is about 1.48, so a 160 bar supply can spike the rod side toward 237 bar under the wrong valve condition. Size the rod-side components — and the seals — for that intensified figure, not the nominal supply.
Cushioning and end-of-stroke loads
A cylinder slamming into its end cap at full velocity delivers a shock load that no static force calculation predicts. Built-in cushions decelerate the piston over the last few millimeters of travel; without them, a fast, heavy load can exceed the mounting and rod-eye ratings on impact. If cycle rates are high or the moving mass is large, specify cushioning and confirm the deceleration force stays within the rod-buckling and mounting limits established above.
Size your cylinder in one screen
The Hydraulic Cylinder calculator takes force, pressure, and geometry and returns bore, rod, forces, and speeds together — the full worked example above, automated.
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