Feature-of-Size (FOS)

In GD&T the term feature-of-size (FOS)  refers to any surface, or set of parallel surfaces associated with a size dimension.  Specific examples of features of size include:

  • A hole diameter (a cylindrical surface)
  • Plate thickness (two opposed parallel surfaces)
  • Peg diameter (another cylindrical surface)
  • A ball bearing diameter (a spherical surface)

The example below shows three views of a plate with two holes.  There are two dimensions called out in this drawing: a plate thickness and a hole diameter.  Either of these is considered a FOS.