Metric bolt grades and strength calculator was developed 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. Mechanical properties of grade 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9 and 12.9 are covered and this calculator was developed according to ISO 898-1 "Mechanical properties of fasteners made of carbon steel and alloy steel - Part 1: Bolts, screws and studs" and ISO 898-7 – Mechanical properties of fasteners -Part 7: Torsional test and minimum torques for bolts and screws with nominal diameters 1 mm to 10 mm standard.

Some important 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 Thread
Property Class


Parameter Symbol Value Unit
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
Minimum --- HV
Maximum ---
Minimum Brinell Hardness
, F = 30 D2
Minimum --- HBW
Maximum ---

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.


Link Usage
Metric Thread Size and Tolerance Calculator Calculates basic major, minor and pitch diameters of the metric external (bolt) thread and internal (nut) thread according to ISO 724:1993 standard. In addition to basic size calculations, tolerance calculations of different tolerance classes can be done according to ISO 965-1:1998 and ISO 965-2:1998 standards.

List of Equations:

Parameter/Condition Symbol 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