Hydraulics explained: pressure, piston size and force
Updated 13 September 2026 · 6 min read
Hydraulics is one of the four areas NEIEP lists for the EIAT's mechanical comprehension section — fitting, since hydraulic elevators use exactly this principle to raise a car. The questions look technical, but they rest on one idea.
The idea: pressure spreads equally
If you push on a liquid trapped in a closed system, the pressure rises by the same amount everywhere in the liquid. This is Pascal's principle. Liquids barely compress, so the push is passed on almost instantly.
Pressure is force spread over an area:
pressure = force ÷ area, so force = pressure × area
Why a small piston can lift a big load
Picture two cylinders joined by a pipe and filled with oil, each with a piston on top. The pressure in the oil is the same under both pistons. But force is pressure times area, so the wider piston gets the bigger force.
Example: a small piston has an area of 2 square inches, and you push on it with 50 lb.
- Pressure = 50 ÷ 2 = 25 psi (pounds per square inch), throughout the oil.
- The large piston has an area of 20 square inches, so it pushes up with 25 × 20 = 500 lb.
The force has been multiplied by ten — the ratio of the two areas.
The catch: the big piston moves less
The oil pushed out of the small cylinder has to go into the large one. The large piston is ten times the area, so the same volume of oil raises it only a tenth as far. Push the small piston down 10 inches and the large one rises 1 inch.
That is the same trade-off as levers and pulleys: more force, less distance. It is why a car jack needs many pumps to lift a car a few inches.
The diameter trap
The single most common mistake in hydraulics questions is treating diameter like area. Area grows with the square of the diameter:
- Double a piston's diameter and its area goes up four times — so does the force it gives.
- Triple the diameter and the area goes up nine times.
If a question gives diameters, square the ratio before comparing forces.
Where you see it
- Car brakes: a light push on the pedal becomes a large clamping force at the wheels.
- Jacks and presses: a narrow pump piston drives a wide lifting piston.
- Hydraulic elevators: fluid pumped under pressure pushes a piston to raise the car.
Practice
Our mechanical practice includes paired-cylinder hydraulics questions asking which piston pushes harder and which moves further.
Frequently asked questions
Why does a wider hydraulic piston produce more force?
The pressure in the fluid is the same everywhere, and force equals pressure times area. A wider piston has more area for the same pressure to act on, so it produces more force.
If you double a piston's diameter, how much does its force change?
It goes up four times, because area increases with the square of the diameter.
Does the larger piston move as far as the smaller one?
No. The same volume of fluid moves both, so the larger piston moves a shorter distance in proportion to how much bigger its area is.
Ready to start?
Try a free sample, then unlock every section's full question bank.