How can I calculate the sum of the perimeter length of all polygon (MultiPolygon) features in a polygon layer in QGIS?
I don't need to know the perimeter of every single polygon feature in this layer, but I need to know the sum of these perimeters for the whole layer. The layer's CRS is non-Earth.
This tutorial "Calculating Line Lengths and Statistics" shows you how to populate the feature length using $length. You can populate the attribute table with $perimeter in the units of measurement for your data CRS using the same methods.
Once your data are populated with perimeter values you can use Basic Statistics for Fields to get the total perimeter value for all your data.
Another option is to use the Statistical Summary tool.
Type $perimiter in the input:
