These three terms appear in almost every piping specification, drawing, and calculation. NPS is nominal pipe size. OD is outside diameter. ID is inside diameter. They describe the same physical pipe from three different reference points, and the question of which one to use at a given moment isn’t a matter of preference — it’s determined by what calculation or decision is in front of you.
Getting NPS, OD and ID explained in terms of when each one applies is more useful than just memorizing the definitions. The definitions are simple; knowing which dimension answers which question is what prevents errors in specifications and calculations.
NPS: the ordering and specification term
NPS is the designation used to specify pipe in project documents and place orders with suppliers. When a drawing calls for “6-inch pipe,” it means NPS 6, and every supplier, contractor, and inspector in the supply chain uses that designation to identify the correct product.
NPS is not an actual physical measurement — it’s a label assigned to a pipe that has a specific outside diameter. For NPS 1/8 through NPS 12, the NPS number is smaller than the actual OD. For NPS 14 and above, NPS equals the OD in inches. Below NPS 14, if you measure the outside diameter of a piece of pipe and it reads 6.625 inches, you order “NPS 6” — not “6.625-inch pipe.” Using the measured OD as the order specification would mean nothing to a supplier working from a standard catalog.
NPS is also the number used to specify fittings, flanges, and valves. An NPS 6 pipe connects to an NPS 6 elbow, an NPS 6 flange, and an NPS 6 gate valve — all designed around the same 6.625-inch outside diameter.
OD: the dimension that controls physical compatibility
Outside diameter is what determines whether two pipe sections or a pipe and fitting will physically connect. All NPS 6 pipe, regardless of schedule, has the same OD: 6.625 inches. Schedule 20, Schedule 40, and Schedule 80 in NPS 6 are all 6.625 inches OD. The wall grows inward as the schedule increases, while the external dimension stays constant. This is what makes fittings interchangeable across schedules — the butt-weld fitting is machined to the NPS OD, not to a specific wall thickness.
OD is the dimension you measure in the field when confirming pipe size. It’s the dimension used in the Barlow pressure formula. It determines insulation jacket sizing, pipe clamp span, and flange face-to-face dimensions. Any calculation or decision that involves the physical space the pipe occupies uses OD.
The relationship between NPS and OD is fixed by ASME B36.10M for carbon steel pipe:
NPS 2: OD 2.375 inches
NPS 4: OD 4.500 inches
NPS 6: OD 6.625 inches
NPS 8: OD 8.625 inches
NPS 10: OD 10.750 inches
NPS 12: OD 12.750 inches
These aren’t approximate. The OD is held to tight dimensional tolerance at the mill.
ID: the flow and process calculation dimension
Inside diameter is what matters when calculating flow rate, pressure drop, velocity, or heat transfer. The bore — the actual opening available to the fluid — depends on both OD (set by NPS) and wall thickness (set by schedule). Unlike OD, the ID varies with schedule for the same NPS:
NPS 6 Schedule 20: ID approximately 6.357 inches
NPS 6 Schedule 40: ID 6.065 inches
NPS 6 Schedule 80: ID 5.761 inches
For the same flow rate, velocity in Schedule 80 is noticeably higher than in Schedule 20, because the bore is smaller. For systems where velocity matters — preventing solids from settling, maintaining turbulent flow for heat transfer, or controlling erosion in abrasive fluid lines — schedule selection directly affects the flow hydraulics, and that connection runs through the inside diameter.
ID is also the starting point when specifying pipe that will receive internal lining. The pipe schedule needs to be selected to provide the required bore after lining, working backwards from the target lined ID to identify which unlined ID is needed, then checking which schedule produces that bore.
Where the three terms get confused
The most common mix-up is using NPS as if it were an actual physical dimension in a flow calculation. NPS 6 is not 6 inches of anything — it resolves to 6.625 inches OD, and then to a schedule-dependent ID. A flow calculation that plugs in 6.000 inches as the pipe diameter for NPS 6 will understate flow area and overstate velocity by about 20%.
The second frequent error is specifying ID in a procurement document as if it fully defines the pipe. Inside diameter alone is ambiguous — it depends on both NPS (which sets the OD) and schedule (which sets the wall). A spec that says “minimum 6.000-inch ID Schedule 40 pipe” is consistent (NPS 6 Schedule 40 has a 6.065-inch ID, which exceeds the minimum). A spec that says “minimum 6.000-inch ID, 6-inch OD” is impossible to satisfy, because NPS 6 Schedule 40 has a 6.625-inch OD.
The practical rule: use NPS for ordering and specification, OD for physical compatibility and pressure calculations, and ID for flow and process calculations. Each has its domain; moving between them requires going through the B36.10M table for OD and the schedule table for ID, not assuming the NPS number carries that information directly.
