News - Steel Pipe Dimensions Decoded: Schedule, Wall Thickness & Size Chart

One of the most confusing aspects when buying steel pipes is the sizing system – OD, wall thickness, nominal size, Schedule… the list goes on. A wrong specification can delay your project or even cause failures on site.

At Royal Steel Group, we’ve put together this quick reference guide based on years of global supply experience. It covers the essentials so you can specify pipes correctly and confidently.

The Three Key Parameters of a Steel Pipe

Any pipe can be described with just three numbers:

Outside Diameter (OD) – the external width of the pipe

Wall Thickness (WT) – how thick the pipe wall is

Length – typically 6 m (20 ft) or 12 m (40 ft)

With these, you can calculate weight, pressure capacity, and cost per meter or foot.

Steel Pipe Dimensions Chart (Size Chart)

Pipe Schedule Chart unit in mm as below, view here for Pipe Schedule Chart in inch.

Click Here

pipe-schedule-chart-inch-royal-steel-group

Industry Standards for Pipe Sizes

Two main standards are used worldwide:

ASME B36.10M – covers dimensions of carbon and alloy steel seamless and welded pipes, for normal, high‑pressure, and high‑temperature service

ASME B36.19M – covers stainless steel pipe dimensions; mostly identical to B36.10M, but some wall thicknesses are marked with an "S" suffix to differentiate

Nominal Sizes: NPS and DN

NPS (Nominal Pipe Size) – the common North American designation, in inches.

A common trap: NPS is NOT the actual OD. For sizes below NPS 12 (DN 300), the actual OD is larger than the nominal number. For example, NPS 4 pipe has an actual OD of 4.5 inches, not 4 inches.

DN (Diameter Nominal) – the metric equivalent used internationally, in millimetres. The DN value is close to, but not exactly, the inside diameter. It ensures that pipes and fittings with the same DN are interchangeable.

Conversion between NPS and DN: DN25 = NPS 1, DN50 = NPS 2, and so on.

Schedule Numbers – The "Grade" of Wall Thickness

Schedule (abbreviated as Sch) is a number that represents a wall‑thickness series – not a fixed thickness value.

For a given nominal size, the higher the Schedule number, the thicker the wall and the higher the pressure rating. Common schedules include Sch 5, 10, 20, 30, 40, 80, 120, and 160.

Why are Sch 40 and Sch 80 so popular? Because they cover the wall thickness needed for the vast majority of industrial applications. They are available in both carbon steel (e.g., API 5L, ASTM A106B) and stainless steel (e.g., ASTM A312 316L), so they are the most widely stocked and used.

Weight Classes (WGT)

Before the Schedule system became standard, wall thickness was classified by weight:

STD (Standard)

XS (Extra Strong)

XXS (Double Extra Strong)

These historical classes correspond to certain Schedule numbers – for exact equivalents, refer to ASME B36.10 and B36.19.

How to Correctly Describe a Pipe Specification

Any of the following formats is acceptable, depending on your preference or regional practice:

Format	Example
OD × Wall Thickness × Length	Φ88.9mm × 5.49mm × 6m
NPS × Schedule	NPS 3″ × Sch 40
NPS × Weight Class	NPS 3″ × STD
OD × Weight per Unit Length	3½″ × 16.8 lb/ft

Royal Steel Group supplies seamless and welded steel pipes to API, ASTM, EN, and JIS standards. If you have any questions about pipe selection, feel free to contact our technical team – we're here to help.