Use the Offset feature in order to calculate volume and area of walls in a much more simple and accurate way rather than measuring it manually. Also, the "Offset" option allows you to calculate GIA and GEA/GSF much faster for polygons.
How to use the option:
The offset feature works interactively.
Choose the relevant polygon or polyline with the help of the Select tool and you will see the offset option appear in the popup menu.
Select it and use the slider to drag left or right to create a contour around the original polygon or polyline.
Also, you are able to choose whether to add offset on both sides or from the single side. Use the central button in the pop-up window to select the appropriate option.
In the pop-up window, you can also see the size of the resulting polygon.
Once all the steps are complete, press the apply button.