Define a custom field function, let’s say called ‘switch’, and set that equal to (1 / 2) * (1 + (0.12 – UDM0) / Abs(0.12 – UDM0)) Notice that this expression is defined this way so that switch will be equal to 1 when UDM0 < 0.12, and equal to 0 when UDM0 > 0.12 Now you can go to Reports > Volume Integrals Report Type = Volume Integral, Field Variable = Custom Field Functions > switch, Select the Cell Zones. Then click on Compute. This will give you the total volume of all cells where UDM0 < 0.12.