Formation Volume Factor

Ratio of fluid volume at subsurface conditions $\begin{array}{l}V_R\end{array}$ to fluid volume at separator conditions $\begin{array}{l}V_S\end{array}$ :

$\begin{array}{l}\displaystyle B = \frac{V_R}{V_S} = \frac{\rho_S}{\rho_R}\end{array}$

where $\begin{array}{l}\rho_S\end{array}$ is density at separator conditions and $\begin{array}{l}\rho_R\end{array}$ is density at subsurface conditions.