It can be defined as pressure per unit length. If the pressure is in psi and length is in feet then the unit of pressure gradient is psi/ft.
Following terms are used in common in context to pressure gradient:
- Hydrostatic pressure gradient
- Formation pressure gradient
If the hydrostatic pressure exerted by a drilling fluid column of density 10 ppg in wellbore at a depth of 1000 feet is 520 psi, then the hydrostatic pressure gradient is defined as the hydrostatic pressure per unit length, which is the ratio of hydrostatic pressure and the depth at which the hydrostatic pressure is considered, ie. 520 psi/1000 ft = 0.52 psi/ft.