How to convert pixel value into mm?

In my algorithm result, I am getting final circle diametr value in pixel I want to convert that into mm.How to go for this.

Answer 1

[Walter Roberson]

You need to have additional information that tells you the conversion between pixels and mm. Most images do not have that information available, and many of the ones that do have it available in the headers are incorrect. You should only trust the headers in the case of calibrated scientific instruments.

The easiest way to proceed is to use the same conditions to take an image of an object with known size, and measure the pixel to size conversion from that.

[Image Analyst]

We really need to put this into the FAQ since we have to answer it about every week.