I know how to draw a circle of a given diameter in pixels on my screen.
If I need to printout this circle onto a A4 paper, and I need this circle be drawn exactly at the page center and to be of an exact diameter in millimeters.
How do I do this?

• Set page accordingly to your desired print size - A4 by default, sounds good
• draw your image with appropriate real world units, preferably change the default unit to like mm in the document's properties (Shift+Ctrl+D)
• save as pdf with approriate settings - output page size: use document's page size by default, getting at it
• open the pdf with a pdf viewer and set up the printing
  -print page size to A4, print to be at actual size (these may also be default for your printer)
• print

If it isn't at the right scale after the process then most likely you need to update your printer's driver and print the exact same pdf again.

thank you very much

you are welcome

I made a circle of 35 mm in diameter.
In pixels it is shown as 124.016
Yet the resulting XML code shows radius of 61.694927
Why not 124.016/2= 62.008 ?
How does Inkscape convert millimeters into pixels?

Currently it works at 90 dpi, future plans changing to 96 dpi.

A viewbox element is added to the document to set the mm/other unit to px globally in an svg.
That may/may not help sorting out what's going on behind.
Like compared to the specs the coordinate system itself is flipped, so at least there is a transformation added too.
I guess if you resize page to selected object/tweak the number of decimals in the settings you could get different results (not a programmer myself).

I see, thank you