Formula for pixel pitch
Sensor width in mm / photosite count across the width.
Or sensor height in mm / photosite count across the height.
Normally they are identical, but not always!
Note that sensor have more photosites than pixels in the images they
produced. For example, the sensor that produces my 3072x2304 images
has 3112 x 2328 photosites.
Not required information, but it might have been educational to explain WHY they make sensors that way...
It's because the Bayer demosaicing algorithms used need "extra" photosites to get started, thus there is a border around the "effective" photosites. These "extra" photosites can sometimes be processed by computer and recovered. Thomas Knoll has a free utility that accepts DNG files and recovers some of the extra photosites in the border. Its called "DNG Recover Edges" and is available from Adobe. Here is a good article on the subject, w/ a link to the utility.