What resolution (pixels/mm) should I use in SVG for CNC?
I found one of those printer things that puts ink on dead trees and tested to print a simple SVG file.

&lt;svg xmlns="http://www.w3.org/2000/svg"
     width="400px" height="800px"&gt;
  &lt;rect x="10" y="10" width="72" height="72" fill="#999999" /&gt;
  &lt;rect x="10" y="100" width="378" height="378" fill="#999999" /&gt;
&lt;/svg&gt;


As I suspected 72 pixels came out pretty much exactly 19mm. (72/19.05)*100~=378 came out 100mm.

Given this I am going to assume that 72/19.05 is the de facto best pixel to mm ratio to use for CNC projects.

EDIT:

Found this documentation: http://w3.org/TR/SVG/coords.html#Units

&lt;svg xmlns="http://www.w3.org/2000/svg"
     width="400px" height="800px"&gt;
  &lt;rect x="10" y="10" width="19.05mm" height="19.05mm" fill="#999999" /&gt;
  &lt;rect x="10" y="100" width="100mm" height="100mm" fill="#999999" /&gt;
&lt;/svg&gt; 


Much simpler to use mm as units right away