Pipe OD, nominal bore and schedules explained

Outside diameter (OD) is measured across the outside of the pipe. Wall thickness t is the thickness of its wall. The inside diameter is approximately OD − 2 × t; tolerances and the actual supplied specification still matter.

Nominal pipe size (NPS) and nominal bore (NB) are size designations. DN is a nominal metric designation, not a measured diameter in millimetres. A nominal inch pipe size must not be converted using 25.4 and then treated as its OD or its exact bore.

For round tube sold by actual outside size, an inch dimension can instead be a real measurement: 1 inch equals 25.4 mm. Check whether the table says nominal size or OD before comparing products.

A schedule identifies a wall-thickness series in a dimensional standard. It is not a wall thickness in millimetres and is not a pressure rating. Select the nominal size and schedule from the relevant table, then read its OD and thickness.

Within a given dimensional series, the OD for the same nominal size stays the same while a thicker wall reduces the bore. The same schedule number does not mean the same thickness at every nominal size.

Do not assume carbon-steel and stainless schedules are interchangeable, or silently substitute a schedule with an S suffix. Ask for the exact material and dimensional standard required by the job.

For pressure service, provide the fluid, working pressure, temperature and required standard to your engineer and sales executive. Material grade, manufacturing method, joints, corrosion allowance and applicable design rules also affect suitability.

A tube that fits a dimension is not automatically suitable for water, gas, steam or structural service. State whether you need seamless or welded pipe, a coating, a particular end connection or a specified test requirement.