EN standards use tolerance classes to define diameter precision. EN 10083-1 (hot-rolled) specifies:
h11: Loose tolerance (±0.13mm for 20mm diameter) for general construction (rebar).
h9: Tighter tolerance (±0.087mm for 20mm) for structural components (columns).
h8: Precision tolerance (±0.052mm for 20mm) for machine parts (shafts).
EN 10277-1 (bright steel) adds stricter classes: h7 (±0.021mm for 20mm) for high-precision parts (bearings) and h6 (±0.013mm for 20mm) for aerospace components. Tolerance classes are marked on the material certificate and round steel labels. Choosing the right class is critical – using h11 round steel for a bearing (needing h7) would cause fit issues, leading to equipment failure.