This report is devoted to the computation of the volume of whispering gallery modes in optical micro-spheres. We first derive the mathematical expression of TE and TM modes in a homogeneous dielectric optical micro-sphere from the general set of Maxwell's equations. Then we present a method to numerically compute the volume of whispering gallery TE or TM modes which do not require any assumptions on the mode numbers as it is usually the case. Last we present the Matlab Toolbox WGMode written from the present study designed to explore resonance conditions, to visualize whispering gallery TE or TM modes in a micro-sphere and to compute the volume of any given mode.