1. For Pipe wall thickness: Use Steel Pipe Schedule, like schedule 40 steel pipe, schedule 80 pipe.
2. For pipe Diameters: Nominal Pipe Size (NPS) and DN (Nominal Diameter)
3. Pipe weight Class (WGT), LB/FT (Pounds per foot), KG/M (Kg per meter)

Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures. Pipe size is specified with two non-dimensional numbers: a nominal pipe size (NPS) based on inches, and a schedule (Sched. or Sch.).

Nominal diameter also means outside diameter. Because as the pipe wall is very thin, the outside and inside diameter of the steel pipe is almost same, so the average value of both parameters is used as the name of the pipe diameter. DN (nominal diameter) is the general diameter of various pipe and pipeline accessories. The same nominal diameter of the pipe and pipe fittings can be interconnected, it has interchangeability. Although the value is close or equal to inside diameter of pipe, it is not the actual sense of the pipe diameter. The nominal size is represented by a digital symbol followed by the letter “DN” and mark the unit in millimeters after the symbol. For example, DN50, a pipe with a nominal diameter of 50 mm.

Nominal diameter DN (mm) and NPS (inch) conversion

1. Conversion DN (mm) that requires separate memory

2. Greater than or equal to DN100 diameter and NPS conversion

DN=25*NPS;
NPS=DN/25

3. Exact conversion

1 inch =25.4 mm

For example: a. Expressed as “pipe outside diameter × wall thickness”, such as Φ 88.9mm x 5.49mm (3 1/2” x 0.216”).  114.3mm x 6.02mm (4 1/2” x 0.237”), length 6m (20ft) or 12m (40ft), Single Random Length (SRL 18-25ft), or Double Random Length (DRL 38-40ft).

b. Expressed as “NPS x Schedule”, NPS 3-inch x Sch 40, NPS 4-inch x Sch 40. Same size as above specification.

c. Expressed as “NPS x WGT Class”, NPS 3-inch x SCH STD, NPS 4-inch x SCH STD. Same size above.

d. There is another way, in North America and South America, usually use “Pipe Outer Diameter x lb/ft” to describe pipe size. As OD 3 1/2”, 16.8 lb/ft. lb/ft is pound per feet.