To find the area of multiple hatches in your drawing, just select those hatches, and check the Cumulative Area in the Properties panel

(use CTRL+1 to open it), as shown in the following picture,

Then we can click the button to pop up the “QUICKCALC” to convert between different units, as shown in the following picture,

Please note:

If we want to know the sum of multiple hatch areas, we have to make sure that each of the hatch area has value itself.