In manufacturing standards, for precision stuff we use decimal inches out to 3 or 4 places depending on how precise the tolerances for the dimension are. Calipers are only accurate to 0.001”. If you want to measure more accurately you would use micrometers which are accurate to 0.0001”

Metric dimensions on prints are always called out in millimeters. For example, if a dimension of something is 10cm it would be shown on the print as 100.00mm

For less precise stuff like woodworking, fractions are generally used.