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This blueprint tries to collect all information about how to implement real world units (in, mm) in a standard compliant way.

The problem

Inkscape side

Even if the user switch everywhere the unit to mm, inkscape convert it to px unit and write it in the .svg file resulting inaccuracies.

SVG side

All the units in svg drawing needs to be in px unit. The grammar in the SVG specification does not allow other units then pixel:

More information about coordinate system:

Examples: This is invalid in SVG:

 <path d="M 0,0 L 30mm,0 L 30mm,30mm L 45mm,30mm" id="path7887" />

The correct solution would be to specify it in pixel and after tranform it.

Related bug reports

 #168753: SVG defaults to pixels (90dpi) despite default units
   #168516: Inkscape doesn't save page size units in plain svg
   #170738: preserve units
 Viewbox related bug reports:
 #171203: Use viewBox to explicitly define a coordinate system

Related discussions in the past

On the inkscape-dev mailing list:

Problem with real units (mm)

Although attaching parasite information to an svg element is not necessary, but maybe useful if additional information must be keeped about conversion inaccuracy. Please see this thread for more information: Attaching parasite information to an object (or group)