By definition, the isothermal compressibility of a substance is defined mathematically by the following expressions:

- In terms of fluid volume:

- In terms of fluid density:

where V and ρ are the volume and density of the fluid, respectively.

For a crude oil system, the isothermal compressibility coefficient of the oil phase co is defined for pressures above the bubble-point by one of the following equivalent expressions:

where

- co = isothermal compressibility, 1/psi
- ρo = oil density lb/ft^3
- Bo = oil formation volume factor, bbl/STB

where Bg = gas formation volume factor, bbl/scf

Ahmed (2001)