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A measure of resistance of a Continuum body to compression.

A measure of isothermal relative change in density $\begin{array}{l}\rho\end{array}$ or molar volume $\begin{array}{l}V_m\end{array}$ under a unit pressure $\begin{array}{l}p\end{array}$ variation:

(1)	$\begin{array}{l}\displaystyle c = \frac{1}{\rho} \left( \frac{\partial \rho}{\partial p} \right)_T = - \frac{1}{V_m} \left( \frac{\partial V_m}{\partial p} \right)_T\end{array}$

Symbol	Dimension	SI units	Oil metric units	Oil field units
$\begin{array}{l}c\end{array}$	M^-1 L¹T²	Pa^-1	kPa^-1	psi^-1

Compressibility depends on the thermodynamic conditions at which it is measured.

The most popular are:

Isothermal Compressibility

Isentropic Compressibility

(2)	$\begin{array}{l}\displaystyle c_T = \frac{1}{\rho} \left( \frac{\partial \rho}{\partial p} \right)_T\end{array}$

(3)	$\begin{array}{l}\displaystyle c_S = \frac{1}{\rho} \left( \frac{\partial \rho}{\partial p} \right)_S\end{array}$

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See also