Steel Pipe Schedule Chart

If you've measured a 25 NB pipe and found it's 33.4 mm across — not 25 mm — you're not losing your mind. Pipe sizes are names, not measurements. Understanding that one fact unlocks the entire pipe sizing system.

This guide explains how DN, NB, NPS, OD, and pipe schedule all fit together — and gives you a complete Australian pipe schedule chart covering NB 6 through NB 300, with wall thickness in millimetres for Schedule 10 through to XXS.

Why pipe doesn't measure what you expect: what "nominal" means

The word nominal means "in name only." A 50 NB (2-inch) pipe does not have a 50 mm OD, a 50 mm ID, or any dimension that measures exactly 50 mm. The OD is 60.3 mm. The NB designation is a legacy carryover from the era when pipe sizes were described by their internal bore — and as wall thicknesses evolved over time, the nominal designation stayed fixed while actual dimensions shifted.

The practical result: pipe sizes are reference numbers, not measurements. Every dimension you actually need — OD, wall thickness, ID — comes from the schedule chart, not from the nominal size label.

DN, NB, and NPS: three names for one system

Three terms are used for nominally the same pipe sizing system, depending on where you are:

• NB (Nominal Bore) — used in Australia, New Zealand, and the UK. The number is in millimetres but does not equal any actual dimension.
• DN (Diamètre Nominal / Diameter Nominal) — the ISO/European metric designation. DN and NB numbers are identical: 50 NB = DN 50.
• NPS (Nominal Pipe Size) — the US imperial designation. NPS is stated in inches. The relationship to DN/NB is not a simple ×25.4 conversion — it's a lookup.

The three systems use different numbers for the same pipe, but the physical dimensions (OD and wall thickness) are defined by the same international standard: ASME B36.10M for carbon and alloy steel, and ASME B36.19M for stainless steel.

DN / NB / NPS conversion table

Note that the DN/NB numbers are not a direct ×25.4 conversion of NPS. 1" NPS = 25 NB (not 25.4), 2" NPS = 50 NB. The numbers are standardised reference designations, not conversions.

What is pipe schedule?

Pipe schedule is a dimensionless number that specifies wall thickness. Given a fixed OD, a higher schedule number means a thicker wall, a smaller internal bore, and a higher pressure rating. The OD does not change between schedules — only the wall and therefore the ID changes.

This is why Schedule 40 and Schedule 80 fittings are compatible with each other: both have the same OD and the same thread form. A 50 NB Schedule 40 nipple and a 50 NB Schedule 80 elbow will thread together without issue, because the OD — and therefore the thread — is identical.

Common schedule designations:

• Sch 10 — light wall, used for low-pressure drainage, sprinkler systems, and vent lines
• Sch 40 / STD — standard wall, the most widely used schedule for water, air, and gas at moderate pressures
• Sch 80 / XS — extra strong, used for higher-pressure applications and threaded connections where wall thinning from threading is a concern
• Sch 160 — heavy wall, used in high-pressure industrial and hydraulic systems
• XXS — double extra strong, the heaviest common designation, used in very high-pressure and high-temperature applications

Steel pipe schedule chart — NB 6 to NB 300 (wall thickness in mm)

All dimensions per ASME B36.10M. Wall thickness shown in millimetres. Weight (kg/m) shown for Schedule 40/STD and Schedule 80/XS — the two most commonly ordered grades.

For NB 6 to NB 250: Schedule 40 = STD, Schedule 80 = XS. For NB 300 and above, Schedule 40 > STD — see note below the table.

NB 300 note: For 300 NB (12") and larger, Schedule 40 and STD (Standard) diverge. STD wall thickness is 9.53 mm; Schedule 40 is 10.31 mm. When ordering or specifying NB 300 and above, always state the schedule explicitly — do not rely on "standard wall."

NB 200 XXS note: For 200 NB, Sch 160 (23.01 mm) is thicker than XXS (22.23 mm). Above NB 150, XXS does not always represent the thickest available wall — verify against the full schedule table for the specific size.

Schedule 40 vs Schedule 80: when to use each

Schedule 40 is the default specification for the majority of industrial, commercial, and domestic piping in Australia. It covers water supply, compressed air at workshop pressures, and natural gas distribution. It is the pipe you will find in most trade supplies under general stock.

Schedule 80 is required when one or more of the following apply:

• Higher operating pressure — Sch 80 carries a higher allowable working pressure than Sch 40 at the same nominal size and temperature.
• Threaded connections in smaller sizes — Threading removes material from the pipe wall. In 15–25 NB (½"–1"), a Sch 40 wall leaves very thin material after threading; Sch 80 provides the additional wall required to thread safely and still meet pressure ratings.
• Corrosive or abrasive service — Where internal erosion or corrosion will thin the wall over time, starting with Sch 80 provides additional service life.
• External mechanical load — Buried pipe, pipe subject to impact, or pipe carrying loads where wall strength matters.

Schedule 40 vs Schedule 80: key dimensions compared

The OD is identical in both columns — confirming that Sch 40 and Sch 80 fittings are physically interchangeable. The difference is entirely in bore and wall.

STD, XS, and XXS explained

STD, XS, and XXS are older weight designations that pre-date the schedule numbering system. They remain in common use, particularly in older Australian specifications and legacy engineering drawings.

• STD (Standard) — equivalent to Schedule 40 for pipe up to and including NB 250 (10"). For NB 300 and above, STD has a thinner wall than Sch 40.
• XS (Extra Strong) — equivalent to Schedule 80 for pipe up to and including NB 200 (8"). For NB 250 and above, XS is equivalent to Schedule 80 only up to this point.
• XXS (Double Extra Strong) — a fixed, very heavy wall with no direct schedule number equivalent for most sizes. Used in high-pressure hydraulic lines, high-temperature steam, and chemical service. For NB 250, XXS equals Schedule 160.

When specifying pipe for fabrication or procurement, use the schedule number rather than STD/XS/XXS for any size NB 200 and above, to avoid ambiguity.

"S" schedules for stainless steel pipe

Stainless steel pipe is specified under ASME B36.19M, which uses "S" suffix schedules: 5S, 10S, 40S, and 80S. These are specific to stainless and should not be confused with the carbon steel schedules in ASME B36.10M.

• 5S and 10S are unique to stainless steel — there are no carbon steel equivalents. They are lighter-wall schedules made possible by stainless steel's higher inherent strength.
• 40S has the same wall thickness as Schedule 40 (B36.10M) for NB 12" and below.
• 80S has the same wall thickness as Schedule 80 (B36.10M) for NB 12" and below.

In Australian practice, stainless pipe in light-duty process and food-grade applications is frequently ordered to Schedule 10S. For pressure systems, 40S is the default. The "S" designation should always appear on the specification to prevent confusion with carbon steel stock.

Application guide: which schedule to specify

These are typical starting points. Final schedule selection for a pressure system must be verified against the design pressure, temperature, fluid, and applicable Australian Standard. For any system operating above 50 kPa, AS 4041 (Pressure Piping) applies. For pipework that requires bending, AS 4041 also sets the wall thinning calculation for the bend extrados — the chosen Schedule must include the thinning allowance to retain rated pressure after bending.

How to specify a pipe correctly

A complete pipe specification includes four elements: nominal bore, schedule, material, and applicable standard. Omitting any one of these creates ambiguity in procurement and fabrication.

Format: [NB] NB × [Schedule] × [Material Grade] to [Standard]

Example: 50 NB × Schedule 40 × Carbon Steel Grade B to ASME B36.10M / ASTM A106M

Example: 100 NB × Schedule 10S × Stainless Steel 316L to ASME B36.19M / ASTM A312M

Common mistake: Specifying only the nominal size and assuming the supplier will default to the correct schedule. "50 NB carbon steel pipe" is an incomplete specification — the supplier may ship any schedule in stock. Always state the schedule.

Worked example: specifying compressed air pipe for a workshop

Scenario: You need to run a compressed air main at 10 bar gauge (1,000 kPa) through a fabrication workshop. Pipe run is 25 NB.

1. Check the schedule chart: 25 NB Schedule 40 (3.38 mm wall) gives an OD of 33.4 mm and an ID of 26.6 mm.
2. Check pressure rating: Per AS 4041, 25 NB Sch 40 carbon steel (Grade B) is rated well above 1,000 kPa at ambient temperature.
3. Check connections: If using threaded fittings at 25 NB, upgrade to Schedule 80 (4.55 mm wall) — threading removes material and Sch 40 is marginal at this size.
4. Specify: 25 NB × Schedule 80 × Carbon Steel Grade B to ASME B36.10M / ASTM A106M.

Frequently asked questions

Why doesn't a 1-inch pipe actually measure 1 inch?

Pipe sizes are legacy nominal designations, not actual measurements. When iron pipe manufacture standardised in the 19th century, the nominal size approximated the internal bore. As wall thicknesses and materials evolved, the OD was standardised for fitting compatibility while the nominal label stayed fixed. A 25 NB (1-inch) pipe has an OD of 33.4 mm and an ID that varies from 26.6 mm (Sch 40) to 24.3 mm (Sch 80). None of these is 25 mm or 1 inch.

What is the difference between DN, NB, and NPS?

DN (Diamètre Nominal) is the ISO/European metric designation. NB (Nominal Bore) is the equivalent designation used in Australia, New Zealand, and the UK. NPS (Nominal Pipe Size) is the US imperial designation in inches. DN and NB numbers are identical — DN 50 = 50 NB. NPS is in a different numbering system: NPS 2" = DN 50 = 50 NB. All three refer to the same physical pipe with the same OD. The actual dimensions are defined by ASME B36.10M regardless of which designation is used.

What does pipe schedule mean?

Pipe schedule is a number specifying wall thickness. For a given nominal bore, the OD is fixed. As the schedule number increases, the wall thickness increases, the ID decreases, the pipe weighs more, and the allowable operating pressure is higher. Schedule 40 is the standard wall for most industrial, plumbing, and HVAC applications. Schedule 80 is the heavy wall used for higher pressures and threaded connections in small sizes.

What is the difference between Schedule 40 and Schedule 80 pipe?

Schedule 40 and Schedule 80 have identical ODs — fittings and threads are interchangeable. The difference is wall thickness. For 50 NB (2-inch) pipe: Sch 40 wall is 3.91 mm, Sch 80 wall is 5.54 mm. The Sch 80 ID is 3.3 mm smaller, and the pipe weighs 37% more per metre. Sch 80 carries a higher pressure rating and is specified for higher-pressure systems, corrosive service, or wherever threaded connections are used in 25 NB and below.

Why is the OD the same for Schedule 40 and Schedule 80 pipe?

The OD is standardised so that fittings, flanges, and threads are compatible across all schedules of the same nominal bore. If the OD changed with schedule, every fitting would need a separate product for each schedule — doubling or tripling inventory. Fixing the OD means a 50 NB elbow fits both Sch 40 and Sch 80 pipe. The wall grows inward, reducing the ID, while the external diameter stays constant.

Are Schedule 40 fittings compatible with Schedule 80 pipe?

Yes, for butt-weld and socket-weld fittings: the OD is the same regardless of schedule, so fittings connect to the pipe correctly. For threaded connections, the thread form is based on OD and is identical. However, Sch 40 fittings have thinner walls than Sch 80 fittings, so in a high-pressure system, the fitting itself — not the pipe — becomes the weak point. For high-pressure threaded systems, specify Sch 80 fittings to match the pipe schedule.

What do STD, XS, and XXS mean on a pipe?

STD (Standard), XS (Extra Strong), and XXS (Double Extra Strong) are older wall-weight designations that pre-date the schedule numbering system. For pipe NB 250 (10") and below: STD = Schedule 40, XS = Schedule 80. For NB 300 (12") and above, STD is thinner than Schedule 40. XXS has no direct schedule equivalent for most sizes — it represents a very heavy wall used in high-pressure and high-temperature service. For modern specifications, use schedule numbers to avoid ambiguity, particularly for NB 200 and above.

What is the difference between Schedule 10 and Schedule 10S?

Schedule 10 (from ASME B36.10M) applies to carbon and alloy steel pipe. Schedule 10S (from ASME B36.19M) applies to stainless steel pipe. In most common sizes, their wall thicknesses are different — 10S is specific to stainless. Always confirm which standard applies before ordering. Carbon steel pipe in Sch 10 and stainless pipe in Sch 10S are not dimensionally identical across all sizes.

What pipe schedule should I use for compressed air?

For workshop compressed air systems up to 15 bar, Schedule 40 carbon steel pipe to ASME B36.10M is the standard specification. For pressures above 15 bar, or for any threaded connections in 15–25 NB, use Schedule 80. All compressed air piping systems in Australia must comply with AS 4041 (Pressure Piping) and AS 3788 (Pressure Equipment — In-Service Inspection), and the system design must be verified by a competent person.

What pipe schedule should I use for water supply?

For domestic and commercial water supply in Australia, Schedule 40 is the standard. For industrial water at moderate pressures (up to 10 bar), Schedule 40 remains the default. In aggressive water chemistries or buried installations where external corrosion is a risk, Schedule 80 provides additional service life. Potable water systems must comply with AS 3500 (Plumbing and Drainage).

How do I convert between DN and NB pipe sizes?

DN and NB are numerically identical — no conversion is needed. DN 50 = 50 NB. The two designations refer to the same pipe in different regional conventions. To convert NPS (US inches) to NB/DN, use a lookup table — the relationship is not a simple ×25.4 multiplication. For example: NPS 1" = 25 NB, NPS 2" = 50 NB, NPS 4" = 100 NB, NPS 6" = 150 NB, NPS 8" = 200 NB.

What does "50 NB Schedule 40" mean?

It means a pipe with a nominal bore designation of 50 mm (equivalent to DN 50 or NPS 2") with a Schedule 40 wall thickness. The actual dimensions are: OD 60.3 mm, wall thickness 3.91 mm, internal bore 52.5 mm, weight 5.44 kg/m. The nominal bore of 50 mm does not correspond to any of these actual measurements — it is a reference designation only. The full specification would be: 50 NB × Schedule 40 × Carbon Steel Grade B to ASME B36.10M.