First, mark the cells with a species mass fraction INSIDE of ISO-MIN and ISO-MAX under ADAPT>ISO-VALUE ADAPTION. Then, split the register in the fluid zone under Mesh->Separate->Cells. Calculate the volume of the resulting new fluid zone under Report->Volume Integrals->Volume. If the split operation fails or you have a number of fluid zones, do the following: 1.Define a custom field function that calculates cell volume 2.Define a UDS 3.Patch the UDS in the register with the custom field function 4.Calculate the volume of the register by Volume Integral->Sum of UDS in the whole fluid zone