text: how to calculate all specifications for new hydraulic power pack.? i am working has fresh engineer i want to purchase new hydraulic power pack how  i can select it your basic selection criteria start with flow rate @ pressure. this is going to depend on what you are trying to do, but you will find a direct correlation between the horsepower input of the pump motor and the combination of flow at pressure. note that it is possible to make low flow at high pressure, or low pressure at high flow, but it takes a lot of power to both.  a suggestion is to calculate the amount of power your require (force x distance / unit of time) and convert that to horsepower, then look for an hpu at www.grainger.com that is about 2x that horsepower.