Plate Thickness Tolerances to EN 10029: Classes A–D and Flatness

The standard and its scope

EN 10029 covers thickness and flatness tolerances for hot-rolled steel plates from 3 mm to 250 mm thick. If a purchase order says nothing about tolerances, class A thickness and class N flatness apply by default — a fact behind many disputes.

The four thickness classes

The classes differ only in how the permitted deviation is distributed around nominal thickness:

Class	Under-thickness (minus) side	Character
A	Depends on nominal thickness (a table value)	The default; the plus side is several times the minus side
B	Fixed −0.3 mm regardless of thickness	Plus side as class A
C	0.0 mm — plate may never be thinner than nominal	Plus side as class A
D	Half the class A band, applied symmetrically ±	Closest to nominal on both sides

Mills roll plate aiming above nominal, so class A's band is heavily asymmetric: a plate ordered at a medium thickness may legitimately measure a couple of millimetres over nominal while only a fraction of a millimetre under. Under class C the same plate can still run thick — only the thin side is eliminated.

Why it matters commercially

• Weight: plate is usually sold by theoretical weight calculated from nominal dimensions — but if you design to minimum thickness, a class A plate at the bottom of its band may be thinner than your calculation assumed.
• Pressure and structural design: design codes use a guaranteed minimum thickness. Specifying class B or C buys certainty that nominal really is the minimum.
• Machining: parts machined from plate need cleanup allowance on both faces; class D keeps the starting thickness predictable.

Flatness: classes N and S

Thickness is not the whole story. EN 10029 also sets flatness tolerances in class N (normal) and class S (special, restricted), measured as the gap under a straightedge over 1 m and over the full plate length. Permitted deviations grow with plate width and shrink with thickness, typically on the order of a few millimetres per metre for medium plates under class N — noticeably tighter under class S. Specify class S for plates going into welded assemblies, laser/plasma cutting beds or machined fabrications where rework on a wavy plate costs more than the tolerance premium.

Specifying tips

• Write the class into the order: "EN 10029 class B" (or C/D) and "flatness class S" if needed — neither is implied by the steel grade.
• Remember EN 10029 governs hot-rolled plate only; cold-rolled sheet and quarto plate to other specifications follow different standards.
• Check the delivered plate at several points: tolerance applies to measurements taken away from the edges, per the standard's measuring rules.