Calculating the volume under an isosurface?

I am using one of the COMSOL tutorial models and I would like to know if there is a way to calculate the volume underneath the isosurface.

I am using the heat transfer radiofrequency ablation model. There is an option to view the isosurface/contours, but I would like to know how to calculate the amount of tissue underneath this isosurface.

Any help at all is much appreciated.