1.Classified by Outer Diameter
Capillary Tubes: Below 10 mm
Small Diameter Pipes: 10.3 mm (1/8") to 102 mm (4")
Medium Diameter Pipes: 114 mm (4.5") to 508 mm (20") or 660 mm (26")
Large Diameter Pipes: Above 508 mm (or 660 mm)
2.Classified by Diameter-to-Wall Thickness Ratio
Generally distinguished by the following ranges:
| Category Diameter | Wall Thickness Wall Thickness | Diameter |
| Extra-Heavy Wall Pipe | ＜5.5 | ＞0.18 |
| Heavy Wall Pipe | 5.5-0.0 | 0.18-0.12 |
| Standard Pipe | 9.1-20 | 0.12-0.05 |
| Thin Wall Pipe | 20.1-50 | 0.05-0.02 |
| Extra-Thin Wall Pipe | ＞50 | ＜0.02 |
3.Classified by Application
(1) Transmission Pipes: Used for the pipeline transmission of water, gas, oil, natural gas, and solids.
(2) Structural Pipes: Used in construction, various steel structures, and furniture manufacturing.
(3) Machinery Manufacturing Pipes: Used in automobile, agricultural machinery, and machine manufacturing.
(4) Boiler and Heat Exchanger Pipes: Used for boiler water tubes, smoke tubes, heater and preheater tubes, heat exchanger tubes in the chemical and petroleum industries, and low-temperature pipes for refrigeration and condensation.
(5) Oil Drilling and Production Pipes: Used for oil drilling rods, casing pipes, etc.
