What does nut strength class mean?

Nut strength class per ISO 898-2 indicates the proof load capability of the nut. It is designated by a single number (e.g., 8, 10, 12) that corresponds to 1/100th of the minimum proof load stress in MPa for the reference bolt-nut combination.

How do I match a nut to a bolt class?

The nut class number should be equal to or greater than the first digit of the bolt class. For example: bolt 8.8 → nut class 8; bolt 10.9 → nut class 10; bolt 12.9 → nut class 12. This ensures the nut threads are at least as strong as the bolt.

What is the proof load stress of a nut?

Proof load stress is the maximum axial stress a nut can sustain without permanent deformation, measured via a hardened mandrel test. For a class 8 nut (≤M16), this is 800 MPa. The actual proof load force depends on the bolt tensile stress area.

Why are nut proof loads size-dependent?

Larger nuts (>M16) have reduced proof load stresses because the nut height to diameter ratio changes, thread engagement efficiency decreases, and load distribution across threads becomes less uniform at larger diameters.

What hardness should a class 8 nut have?

A class 8 nut per ISO 898-2 should have Vickers hardness HV 200–302 (approximately Rockwell HRC 20–30). Nuts that are too hard may be brittle; too soft may strip threads under load.

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Nut Strength Class — ISO 898-2

Look up ISO 898-2 nut classes: proof load stress, hardness range, and matching bolt property class. Select nut class and size.

ISO 898-2Classes 5–12M5 – M64

Nut Class

Bolt/Nut Size

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Results

Proof Load Force

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Proof Load Stress (Sp)

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Hardness Range (HV)

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Matching Bolt Class(es)

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Tensile Stress Area (As)

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Proof Load

The proof load of a nut is the maximum axial force it can withstand without permanent deformation:

ISO 898-2 Nut Class Summary

Nut Class	Sp (MPa) ≤M16	HV Range	Matching Bolt
5	500	130–302	4.6, 4.8, 5.6, 5.8
6	600	150–302	6.8
8	800	200–302	8.8
10	1040	260–353	10.9
12	1220	295–353	12.9

⚠️ Note: For sizes >M16, proof load stresses are reduced per ISO 898-2 Table 2. Always check the standard for the actual size-dependent values.

Nut Strength Class ISO 898-2 Lookup | Free Online Tool

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Results

Proof Load

ISO 898-2 Nut Class Summary