My metric bolt measures .780 with calipers over the threads what size is it

Bolts are usually classified in terms of two types of measurements: the metric system and the imperial system. In calipers, the metric system is in units of millimeters, while the imperial system is in units of inches. For example, when a bolt is sized as 12 × 1.25 in the metric system, that means the bolt thread diameter measures 12 mm, and the thread pitch measures 1.25 mm.

So, if your metric bolt measures 0.78 over the threads, that means that the size of the bolt is 0.78 mm in thread pitch. The attached picture shows how you measure the thread pitch.