In order to create a new Volume:
1. On the Menu bar, click View > Volumes.
2. On the Menu bar, click Volumes > New Volume.
3. On the 3D View, a green point appears beside the mouse. Left click to mark the vertices of the base surface of the volume. With each click a vertex is created and the volume base surface is formed.
4. Right click to add the last vertex and to create the volume base.
5. (optional) To mark the vertices more accurately, on the 3D View, select the volume and correct the position of the vertices: How to edit a Volume.
6. (optional) In the sidebar, click to open the Volume Settings pop-up and select how the Base Surface will be defined. For more information: Menu View > Volumes > Sidebar > Objects.
7. On the sidebar, click Compute to compute the volume.