Hydrostatic pressure is pressure exerted by density of fluid column. The relationship of hydrostatic pressure is shown in the equation below.
HP (Hydrostatic Pressure) = density x g (gravity acceleration) x h (True Vertical Depth, TVD)
In oilfield term, the formula above is modified so that people can use it easily. The formulas are as follows:
HP = Constant x MW x TVD
HP (psi) = 0.052 x MW (ppg) x TVD (ft) ** Most frequent used in the oilfield **
HP (psi) = 0.007 x MW (pcf) x TVD (ft)
HP (kPa) = 0.00981 x MW (kg/m3) x TVD (m)
According to the equation, Hydrostatic Pressure is not a function of hole geometry. Only mud weight and True Vertical Depth (TVD) affect on Hydrostatic Pressure. For example (a picture below); well A and well B have the same vertical depth. With the same mud density in hole, the bottom hole pressure due to hydrostatic pressure is the same. The only different between Well A and Well B is mud volume.
This concept is basic and very important for many aspects such as well control, balance cementing, u-tube, etc.
Reference book: Well Control Books