Tie line width to map scale by a formula?

Question

I will explain it by an example :
In scale 8000 the width of a road is 2 mm = 16 map units.
If I use the unit mm, then width at any scale will be 2 mm. In scale 32000 I find a width of 2 mm is too much.
If I instead use the unit map units, then the width at any scale will be 16 map unis. In scale 32000 the width will be 16 map units = 0,5 mm, which I find too small.
In scale 32000 I want a width of 1 mm. This I could get, if I could calculate the width by a formula :
width = sqrt(32000/scale) mm
Is that possible in QGIS ?

Answer 1

You can build an expression in the line width setting that would look like this:

sqrt(32000/@map_scale )

@map_scale is a variable in the expression builder, and sqrt is a function available in there as well.