How to convert total number of pixels into centimeter square?

How to convert total number of pixels into centimeter square or inch square?

Answer 1

[Jonas]

without further knowledge it is not possible. imagine having a photo of an landscape or of your own hand, the image has same size but the sizes differ heavily. the only possibility to restore real size is if you have some kind if landmarks from which you know size/distance. from those you can maybe also conpensate for distortion originating from perspective view etc

[Walter Roberson]

In some cases you can use header information in the image to make calculations. However, unless the image was taken with a scientific instrument or a Pro camera, most of the time of the header information exists at all, it is an outright lie.