3/8 NPT is about .545" ID, 1/2 NPT is about .674" ID.
Not necessarily. NPT size definitions do not refer to a physical dimension which always confuses my new apprentices. For example a 1/2" male NPT thread on a pipe will have the following inside diameter based on pipe schedule:
schedule 10 = .674" I.D. (.083" wall)
schedule 40 = .622" I.D. (.109" wall)
schedule 80 = .546" I.D. (.147" wall)
schedule 160 = .466" I.D. (.187" wall)
