How to read a pipe schedule chart

NPS and DN are designations, not measurements

Nominal Pipe Size (NPS) in inches and Diametre Nominal (DN) in millimetres are naming conventions. For NPS 2 and below the label does not match any physical dimension of the pipe; from NPS 14 upward the NPS number equals the outside diameter in inches.

• NPS 1/2 = DN 15, NPS 2 = DN 50, NPS 8 = DN 200
• NPS 2 pipe has an outside diameter of 60.3 mm, not 50.8 mm
• NPS 14 and above: OD in inches equals the NPS number

Outside diameter is fixed; schedule changes the wall

For a given NPS the outside diameter is constant across all schedules so that the same fittings, flanges and tooling fit. Increasing the schedule number thickens the wall inward, which reduces the bore and increases weight per metre.

• SCH 10, 40, 80, 160 and XXS all share the same OD for a given NPS
• Higher schedule = thicker wall, smaller inside diameter, higher weight
• Inside diameter = OD − (2 × wall thickness)

Which standard applies

ASME B36.10M covers welded and seamless wrought steel pipe dimensions; ASME B36.19M covers stainless steel pipe and introduces the S-suffix schedules such as 10S, 40S and 80S. Always confirm the edition and schedule against your project specification.

What to send with an enquiry

A complete piping enquiry usually names size, schedule or wall thickness, material grade, end preparation, quantity and any documentation required. Sending all of these at once avoids a round of clarification.

• Size as NPS or DN, plus schedule or wall thickness in mm
• Material grade and applicable specification
• End type: plain end, bevelled end or threaded
• Quantity in metres or pieces, and the delivery location

Common questions

Is DN 50 the same as 2 inch pipe?

Yes. DN 50 is the metric designation for NPS 2. Both refer to pipe with an outside diameter of 60.3 mm.

What is the difference between SCH 40 and SCH 80?

For the same nominal size, SCH 80 has a thicker wall than SCH 40. The outside diameter is identical, so SCH 80 has a smaller bore and a higher weight per metre.

What does the S mean in 40S?

The S-suffix schedules come from ASME B36.19M and apply to stainless steel pipe. In some sizes 40S and 40 have the same wall thickness, but not in all sizes, so check the table.