Pipe Schedule Chart: NPS, OD, Wall Thickness & Flow Capacity Explained

Pipe schedule is one of the most misunderstood concepts in piping engineering. Engineers often assume "schedule" refers to diameter, but it actually refers only to wall thickness. Two pipes of the same schedule but different NPS sizes will have very different wall thicknesses.

Understanding pipe schedules correctly is essential for pressure vessel and piping design, material procurement, welding procedure qualification, and cost estimation. An incorrect schedule selection can mean over-specifying (wasting material cost) or under-specifying (creating a safety hazard).

NPS stands for Nominal Pipe Size — it is a North American standard (ASME B36.10 for carbon steel, ASME B36.19 for stainless steel) that identifies pipe by a nominal (approximate) diameter.

Important: For NPS 1/8 through NPS 12, the Outside Diameter (OD) does NOT equal the nominal size. Above NPS 14, the OD equals the nominal size in inches.

Why the disconnect? The NPS system was developed around standard thread sizes, which were in turn based on internal flow area. The OD was standardised so that a given NPS always uses the same threading dies and fittings, regardless of wall thickness.

The practical implication: If you want the actual internal diameter (ID) — for flow calculations — you must know both the OD (fixed per NPS) and the wall thickness (varies by schedule).

ID = OD − (2 × Wall Thickness)

The most common schedule designations in industrial piping are 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, 160, STD, XS (Extra Strong), and XXS (Double Extra Strong).

STD (Standard) = Schedule 40 for NPS ≤ 10; differs from Sch 40 above NPS 10. XS (Extra Strong) = Schedule 80 for NPS ≤ 8; differs from Sch 80 above NPS 8. XXS = No schedule number equivalent — thicker than Sch 160.

Schedule 40 is the most common choice for general utility service (water, compressed air, low-pressure steam). Schedule 80 is used for higher pressures and corrosive services. Schedule 160 and XXS are used in high-pressure hydraulics, chemical plants, and refineries.

Dimensions per ASME B36.10M for carbon steel pipe. All dimensions in millimetres (mm). Flow ID calculated from OD − 2×WT.

Pipe weight is critical for structural support design, transportation planning, and material cost estimation.

Weight per metre (kg/m) = 0.02466 × Wall Thickness (mm) × [OD (mm) − Wall Thickness (mm)]

This formula applies to carbon steel (density 7,850 kg/m³). For stainless steel (density 8,000 kg/m³), multiply by 8000/7850 = 1.019.

Example: 4-inch Schedule 40 carbon steel pipe OD = 114.3 mm, WT = 6.02 mm Weight = 0.02466 × 6.02 × (114.3 − 6.02) = 0.02466 × 6.02 × 108.28 = 16.06 kg/m

For a 6-metre spool, this is 96.4 kg — important to know when designing pipe supports spaced at 3 m intervals (each support must carry approximately 48 kg of pipe, plus the weight of fluid inside).

The maximum allowable working pressure (MAWP) of a pipe depends on wall thickness, pipe material, and temperature. The governing equation is derived from the ASME B31.3 code:

P = (2 × S × T × E) ÷ (D − 2 × Y × T)

Where: P = Internal design gauge pressure (MPa) S = Allowable stress of pipe material at design temperature (MPa) T = Pipe wall thickness (mm) E = Longitudinal joint efficiency (1.0 for seamless pipe) D = Outside diameter (mm) Y = Material-dependent coefficient (typically 0.4 for ductile materials at temperatures below 480°C)

For carbon steel A106 Gr B (S = 138 MPa at 38°C) in 2-inch Schedule 40 pipe: P = (2 × 138 × 3.91 × 1.0) ÷ (60.3 − 2 × 0.4 × 3.91) P = 1079.16 ÷ (60.3 − 3.128) P = 1079.16 ÷ 57.172 = 18.9 MPa ≈ 189 bar

Note: This is the calculated pressure capacity. The actual MAWP in a piping system is also limited by the weakest component in the system — fittings, flanges, valves, and welds.

Follow this decision framework when specifying pipe schedule for a new application:

Once you know the internal diameter, calculating pipe volume and flow capacity is straightforward.

Pipe Volume = π × (ID/2)² × Length

For flow rate using the Hazen-Williams equation for water: Q = 0.2785 × C × D^2.63 × S^0.54

Where Q is in m³/s, C is the Hazen-Williams roughness coefficient (130 for commercial steel, 100 for older steel), D is the internal diameter in metres, and S is the hydraulic slope (head loss per unit length).

For industrial applications, use our free Pipe Volume Calculator and Flow Rate Calculator, which handle both imperial and metric inputs.

Outside North America, pipe sizes are typically specified using DN (Diameter Nominal) per ISO 6708. The relationship between DN and NPS is:

DN 15 = NPS ½ DN 25 = NPS 1 DN 50 = NPS 2 DN 100 = NPS 4 DN 200 = NPS 8 DN 300 = NPS 12

The OD values are identical between DN and NPS for standard sizes — only the designation system differs. Schedule numbers are the same for carbon steel (ASME B36.10) and stainless steel (ASME B36.19) in both systems.