Pipe schedules explained and how to match fitting wall thickness

Ask three people what a schedule 40 pipe is and you will usually get three partly correct answers. Pipe schedules are simple once the system is clear, and getting it right is essential for matching fittings to pipe.

NPS is not the outside diameter

Nominal pipe size (NPS) is a label, not a measurement. For sizes from NPS 1/8 to 12, the actual outside diameter is larger than the nominal number, so a 2 inch pipe has an outside diameter of about 60.3 mm. From NPS 14 upward, the outside diameter equals the nominal size in inches. The outside diameter for a given NPS stays fixed regardless of schedule, so thicker walls reduce the inside diameter.

What a schedule is

A schedule is a number that sets wall thickness for a given NPS. ASME B36.10M covers welded and seamless wrought steel pipe in carbon and alloy steel, with schedules such as 10, 20, 30, 40, 60, 80, 100, 120, 140 and 160. A higher schedule means a thicker wall and a higher pressure capability. The same schedule number gives a different wall thickness at different sizes, so Schedule 40 in 2 inch pipe is thinner in absolute terms than Schedule 40 in 12 inch pipe.

STD, XS and XXS

Older designations remain in use. Standard (STD) matches Schedule 40 up to NPS 10, extra strong (XS) matches Schedule 80 up to NPS 8, and double extra strong (XXS) is thicker than Schedule 160 in most sizes. Above those sizes, STD and XS become fixed wall thicknesses that no longer match a schedule number.

Stainless steel schedules

ASME B36.19M covers stainless steel pipe, with schedules 5S, 10S, 40S and 80S. Stainless pipe has thinner walls in the lower schedules because of its corrosion resistance and strength, and 10S is widely used. For many sizes 40S and 80S equal STD and XS. Do not assume a carbon steel schedule 10 equals a stainless 10S, as the walls can differ at some sizes.

Why fittings must match the pipe

A buttweld fitting is welded end to end with the pipe. The ends are bevelled, normally at 37.5 degrees to ASME B16.25, and the joint depends on the two walls being about the same thickness. If they differ a lot, the weld has to be built up or tapered, flow is disturbed and stresses concentrate at the joint. For that reason you specify the fitting with the same schedule or wall thickness as the connecting pipe.

A worked example

A line designer specifies a 4 inch schedule 40 carbon steel pipe with long radius elbows and a reducer to 2 inch schedule 40. The enquiry should list the 4 inch Sch 40 LR elbows, and the 4 x 2 inch concentric reducer with 4 inch Sch 40 at the large end and 2 inch Sch 40 at the small end. Leaving the schedule out of the reducer line is one of the most common enquiry omissions.

Quick reference

What to put on an enquiry

• NPS and schedule or exact wall thickness in millimetres
• Whether the schedule is a B36.10M or B36.19M schedule
• Both schedules for reducing fittings
• Any special wall, such as a thicker wall for corrosion allowance

Send us your pipe list with sizes and schedules, and we will quote matching buttweld fittings. Tell us the wall you need and we will confirm the fitting wall.