Drawing grid in Inkscape in millimeter

If I try to draw a grid in Inkscape, there are two different options:

1. Extra -> Render -> Grid


2. Extra -> Render -> Kartesian Grid

But both of them use pixel-metric(?).

Is it possible to use other metrics (e.g. millimeter).

As a workaround, I converted pixel->millimeter, but it didn't work. There is always an odd scale factor in the conversion :-(.

Do you have any solution?