Nominal pipe size is a name, not a measurement. Each NPS designation corresponds to one fixed outside diameter no matter which schedule the pipe is made in — a 2-inch pipe is always 2.375 in OD, a 1-inch is 1.315 in — and none of the common sizes actually measures its nominal number anywhere. The schedule then thickens the wall inward from that fixed OD, which is the whole logic of the system: fittings interchange across schedules because the outside never changes, while the bore, and with it the flow capacity, shrinks as the schedule climbs.
The bore is therefore always arithmetic, never a catalog number: ID = OD − 2 × wall. For steel the ODs and walls come from ASME B36.10M; PVC pressure pipe under ASTM D1785 uses the same iron-pipe-size ODs with its own minimum walls. Copper tube opts out of the convention entirely — its OD is always the nominal size plus 1/8 in, and wall thickness is set by a type letter (K heaviest, then L, then M) rather than a schedule number.