ISO 657 (Hot-rolled steel sections) is an ISO standard that specifies the tolerances for hot-finished circular, square and rectangular structural hollow sections and gives the dimensions and sectional properties for a range of standard sizes.

This first edition as an International Standard constitutes a technical revision of ISO Recommendation R 657-1:1968. ISO 657 consists of 21 parts integrating any shapes of sections. ISO 657-1 specifies dimensions of hot-rolled equal-leg angles.