Note that plane-angle is not an official metric (SI) dimension
millimeter
has dimensions of
length
If both the units have same dimensions then only they can be inter changes..
and as radian is from plane angle and mm is from length they cann't be converted
you need for this question to determine radius of circle then if multiple radius to angle value you have quantity with length dimension as dimension of radius.
jsps,
Radian ia an angular quantity and is dimensionless. If you are interested in determining the arc length (l) on a circle subtended by an angle in radians, use the formula l = wr, where r is the radius, w is the radian angle.
Regards,
Kral
It's exactly like someone asking how to convert amperes into tomatoes.
TOTALLY INCOMPATIBLE UNITS!
To do a conversion between mm and radians, you need a circle of a known radius (example, using a circle with a 1 meter radius, a radian would measure 1000mm) which is NOT the case in the JSPS's question.
An arc characterized by an angle of A radian and having a radius of R meter has a length equal to AxR meter.
Your question was not too complicated to be misunderstood.