Metric bolt grades and strength calculator to show proof strength, tensile strength, Vickers / Brinell hardness, minimum breaking torque and stress area of metric bolts and screws made of carbon steel and alloy steel with designations 4.6, 4.8, 5.8, 8.8, 9.8, 10.9 and 12.9.

Some notes from ISO 898 metric screw threads standards:

- Standard specifies the mechanical properties of bolts, screws and studs made of carbon steel and alloy steel when tested at an ambient temperature range of 10 °C to 35 °C. Fasteners conforming to the requirements of ISO 898 are used in applications ranging from −50 °C to +150 °C.

- ISO 898 applies to bolts, screws and studs with coarse pitch thread M1.6 to M39 and fine pitch thread M8 X 1 to M39 X 3 with triangular ISO thread

- It does not apply to set screws and similar threaded fasteners

- The minimum breaking torques are not valid for hexagon socket set screws.

Metric Bolt Grades and Strength Calculator:

Metric Thread
Property Class


Parameter Value
Designation --- ---
Property Class ---
Screw Thread Series ---
Nominal Stress Area [As_nom] --- mm2
Minimum Tensile Strength [Rm_min] --- MPa
Minimum Ultimate Tensile Load --- N
Minimum Stress at 0,2 % non-proportional elongation [Rp0.2_min] --- MPa
Stress Under Proof Load [Sp] ---
Proof Load --- N
Minimum Breaking Torque [MB_min] --- N.m
Vickers Hardness
, F ≥ 98 N
Min. --- HV
Max. ---
Minimum Brinell Hardness
, F = 30 D2
Min. --- HBW
Max. ---

Note: Breaking torque test applies to bolts and screws with thread less than M3 for which no breaking and proof loads are indicated in ISO 898-1, as well as to short bolts and screws with nominal diameters 3 mm to 10 mm which cannot be subjected to a tensile test. Minimum breaking torques are valid for bolts and screws with the thread tolerances 6g, 6f and 6e.


Alloy Steel: A ferrous (or iron-based) alloy that contains appreciable concentrations of alloying elements (other than C and residual amounts of Mn, Si, S and P). These alloying elements are usually added to improve mechanical and corrosion resistance properties.

Proof Strength: Specified loads or, in the case of proof stresses, specified stresses that the fastener must withstand without any permanent deformation.


List of Equations:

Parameter/Condition Equation
Nominal Stress Area [As_nom] As_nom = (π/4) * ((d2 + d3) / 2)^2
Minimum Breaking Torque [MB_min] MB_min = τB_min * Wp_min
Polar Moment of resistance [Wp] Wp_min = (π/16) * d3_min^3
Torsional Strength [τB] τB_min= X * Rm_min
Strength Ratio [X] Grade 8.8 --- X = 0.84
Grade 9.8 --- X = 0.815
Grade 10.9 --- X = 0.79
Grade 12.9 --- X =0.75


  • ISO 898-1:2013– Mechanical properties of fasteners made of carbon steel and alloy steel Part 1: Bolts, screws and studs with specified property classes – Coarse thread and fine pitch thread
  • ISO 898-7:1992 – Mechanical properties of fasteners - Part 7: Torsional test and minimum torques for bolts and screws with nominal diameters 1 mm to 10 mm