Bolt Property Class & Grade Chart

• Proof stress is what the bolt must carry in a test and show no lasting stretch. Tightening targets are set from this.
• Yield strength is where permanent stretch begins, at 0.2% for high-strength classes.
• Tensile strength is the most the bolt can carry before it breaks.

The class name gives nominal values, and the table gives the minimums. Class 10.9 reads as 1,000 × 0.9 = 900 MPa nominal yield, and its specified minimum is 940 MPa. Neither figure is its proof stress, which is 830 MPa. Mixing these up overstates the safe clamp load by about 8%.

Multiply the stress by the tensile stress area of the thread, which the thread stress area calculator gives.

• M16 class 8.8. Area 157 mm². Proof load = 157 × 580 = 91.1 kN. Minimum breaking load = 157 × 800 = 125.6 kN.
• M20 class 8.8. Area 245 mm², and the higher values apply above M16. Proof load = 245 × 600 = 147 kN.
• 1/2-13 Grade 5. Area 0.1419 in². Proof load = 0.1419 × 85 = 12.1 kip.

A bolt is usually tightened to about 75% of proof load, which is what the bolt torque chart assumes. To check a bolt with a real load, use the bolt stress calculator.

The strengths line up closely, but the parts are not interchangeable. Threads and head sizes differ, so a like-for-like swap means changing the bolt, the nut and often the hole.

A marking gives strength, not corrosion resistance or a temperature rating. High classes also bring trouble of their own. Class 12.9 is more sensitive to hydrogen embrittlement, so coatings and cleaning need care. Nuts must match: class 8 for 8.8, class 10 for 10.9 and class 12 for 12.9.

Structural steelwork follows its own bolting specification, so use the grade on the drawing. The fastener strength calculator compares bolt grades under shear and tension.