Core idea
Pressure acting over piston area produces force. Real output is lower because of friction, pressure loss and return-side pressure.
Practical approach
Confirm units, identify the governing standard or manufacturer data, record assumptions, calculate an initial value, and check whether the result is sensitive to friction, temperature, efficiency, tolerances or material variation.
Use calculators with judgement
Online tools are useful for comparison and first-pass design. Safety-critical work requires suitable factors of safety, verified inputs and review by a competent person.