You can use Isovolumes and the Function Calculator to do this. Create a Volume (Insert > Location > Volume) and select “Isovolume” under Method. Choose the desired Variable and Mode. You can limit the display to only show regions that have a variable value above a certain level by setting the Mode to “Above Value” and specifying a value. Once the Isovolume has been created, go to the Calculators tab and select Function Calculator. Select “volume” under Function and the Isovolume you have created under Location. Click on the Calculate button to show the volume of regions where the variable is above the specified value.