This page provides mechanical strength data for metric bolts, screws, studs and nuts. Carbon and alloy steel fasteners are covered by the ISO 898 series, while corrosion-resistant stainless steel fasteners are covered separately by the ISO 3506 series.
The tables provide quick-reference values for common property classes and include nominal tensile strength, minimum tensile strength, yield strength and proof stress where applicable.
See also: Fastener Engineering Tables and Design Data.
Metric bolt property classes such as 8.8, 10.9 and 12.9 identify specified mechanical properties of carbon and alloy steel fasteners.
The following values are given in N/mm2, numerically equivalent to MPa.
| Property Class | Nominal Tensile Strength Rm,nom |
Minimum Tensile Strength Rm,min |
Lower Yield Strength ReL,min |
0.2% Non-Proportional Elongation Stress Rp0.2,min |
|---|---|---|---|---|
| 3.6 | 300 | 330 | 180 | – |
| 4.6 | 400 | 400 | 240 | – |
| 4.8 | 400 | 420 | 320 | – |
| 5.6 | 500 | 500 | 300 | – |
| 5.8 | 500 | 520 | 400 | – |
| 6.8 | 600 | 600 | 480 | – |
| 8.8 – d ≤ 16 mm | 800 | 800 | – | 640 |
| 8.8 – d > 16 mm | 800 | 830 | – | 660 |
| 9.8 | 900 | 900 | – | 720 |
| 10.9 | 1000 | 1040 | – | 940 |
| 12.9 | 1200 | 1220 | – | 1100 |
The mechanical properties applicable to a particular fastener can depend on nominal diameter, thread type and the requirements of the relevant product standard. The applicable ISO 898-1 tables should therefore be checked for final design or specification.
For the common ISO property-class designation consisting of two numbers, such as 8.8, the designation provides a convenient indication of nominal tensile and yield-related strength.
The nominal tensile strength is:
where x is the first number in the property-class designation.
The nominal yield-strength ratio is:
where y is the second number in the property-class designation.
For property class 4.6:
The nominal yield-strength ratio is:
Therefore:
Nuts use a different property-class system from externally threaded fasteners. Their mechanical properties are specified by ISO 898-2:2022.
ISO 898-2:2022 covers nuts with property classes 04, 05, 5, 6, 8, 10 and 12, including proof-load requirements. The suitability of a nut for a particular bolt property class depends on the nut style, thread diameter and the requirements of the standard.
| Nut Property Class | Typical Mating Bolt Property Class |
|---|---|
| 5 | Up to property class 5.8 where permitted by ISO 898-2 |
| 6 | Up to property class 6.8 where permitted by ISO 898-2 |
| 8 | Up to property class 8.8 where permitted by ISO 898-2 |
| 10 | Up to property class 10.9 where permitted by ISO 898-2 |
| 12 | Up to property class 12.9 where permitted by ISO 898-2 |
Corrosion-resistant stainless steel fasteners are not covered by the carbon and alloy steel property classes of ISO 898-1. Their mechanical properties are specified separately by ISO 3506-1:2020.
The stainless fastener designation combines a stainless steel grade with a property class. For example, A4-80 identifies an austenitic A4 stainless steel fastener with property class 80.
| Group | Designation | General Description |
|---|---|---|
| Austenitic | A1, A2, A3, A4, A5 | Chromium-nickel and chromium-nickel-molybdenum stainless steels widely used where corrosion resistance is required. Austenitic grades are generally non-magnetic in the solution-annealed condition but can become partly magnetic after cold working. |
| Martensitic | C1, C3, C4 | Hardenable stainless steels capable of relatively high mechanical strength. Their corrosion resistance is generally lower than that of common austenitic stainless steels. |
| Ferritic | F1 | Chromium stainless steels with magnetic behaviour and mechanical and corrosion properties differing from the austenitic grades. |
The following table retains the commonly used stainless fastener strength classes as a quick engineering reference. Values are minimum tensile strength Rm and minimum 0.2% proof stress Rp0.2 in N/mm2.
| Stainless Group | Grade / Type | Property Class | Minimum Tensile Strength Rm (N/mm2) |
Minimum 0.2% Proof Stress Rp0.2 (N/mm2) |
|---|---|---|---|---|
| Austenitic | A1, A2, A3, A4, A5 | 50 | 500 | 210 |
| 70 | 700 | 450 | ||
| 80 | 800 | 600 | ||
| 100 | 1000 | 750 | ||
| Martensitic | C1 / C4 | 50 | 500 | 250 |
| C1 / C4 | 70 | 700 | 410 | |
| C1 | 110 | 1100 | 820 | |
| Martensitic | C3 | 80 | 800 | 640 |
| Ferritic | F1 | 45 | 450 | 250 |
| 60 | 600 | 410 |
The two designation systems should not be confused. A carbon/alloy steel 8.8 bolt and a stainless steel A4-80 bolt may have similar minimum tensile-strength magnitudes, but they are specified under different standards and have different proof stress, material, manufacturing and corrosion-resistance requirements.
Fastener selection should therefore be based on the complete mechanical, environmental and joint requirements rather than tensile strength alone.
Use these calculators to apply fastener strength and stress data to practical engineering checks.
| Calculator | Use |
|---|---|
| Bolted Joint and Fastener Calculators | Main hub for RoyMech bolt stress, preload, thread and layout calculators. |
| Bolt Tensile Stress Calculator | Calculate tensile stress from axial bolt load and tensile stress area. |
| Thread Tensile Stress Area Calculator | Calculate ISO metric thread tensile stress area from nominal diameter and pitch. |
| Bolt Proof Load Calculator | Calculate bolt proof load from proof stress and tensile stress area. |
| Bolt Shear Stress Calculator | Calculate single-shear and double-shear stress in a bolt. |
| Bolt Combined Stress Calculator | Assess combined tensile and shear stress using the von Mises criterion. |
The mechanical properties specified by these standards apply under their defined test conditions. Elevated or low temperatures, corrosion, hydrogen embrittlement, coatings, fatigue, special materials and safety-critical applications may require additional assessment or other standards.
Standards are revised periodically. Confirm the current edition and the requirements of the applicable product and application standards before final design.