ERW (Electric Resistance Welded) pipes are widely used in construction, oil & gas, water transmission, and structural applications. When selecting ERW pipes, outer diameter (OD) and wall thickness are the two most critical dimensional parameters, as they directly determine pressure capacity, mechanical strength, pipe weight, and compliance with international standards. This guide explains ERW pipe OD and wall thickness specifications, covering size systems, wall thickness series, material limitations, applicable standards, and selection considerations.