Bolt Grades Explained: Metric Bolt Strength Chart (8.8, 10.9, 12.9)

What Are Bolt Grades?

Bolt grades indicate the mechanical strength of a fastener, helping engineers and installers determine how much load a bolt can withstand before deforming or failing.

Metric bolts use a two-number strength classification system, such as:

• 8.8

• 10.9

• 12.9

These numbers represent the bolt’s tensile strength and yield strength, measured in megapascals (MPa).

Understanding bolt grades is critical when designing or assembling:

• structural steel connections

• heavy machinery

• construction equipment

• mining equipment

• automotive assemblies

Selecting the correct bolt grade ensures the fastener provides sufficient clamping force and structural reliability.

Metric Bolt Grades Chart

Higher numbers indicate greater bolt strength and load capacity.

How Metric Bolt Grades Are Calculated

Metric bolt grades follow a standard formula used internationally in engineering.

R_m = a \times 100

Where:

• Rₘ = tensile strength (MPa)

• a = first number in the bolt grade

Example:

Grade 8.8

Yield strength is calculated as:

Example:

Metric Bolt Grades Explained

Grade 4.6 Bolts

Grade 4.6 bolts are low-strength fasteners used for light-duty applications.

Common uses:

• furniture

• light brackets

• sheet metal assemblies

• non-structural applications

These bolts are typically made from mild carbon steel.

Grade 8.8 Bolts (High Tensile)

Grade 8.8 bolts are the most common high tensile bolts used in construction and industrial applications.

They provide a strong balance of strength, durability and cost efficiency.

Typical applications:

• steel fabrication

• industrial machinery

• construction assemblies

• automotive components

Many hex bolts and structural bolts are manufactured in this grade.

Grade 10.9 Bolts

Grade 10.9 bolts are high-strength alloy steel fasteners designed for demanding mechanical environments.

Common uses include:

• mining equipment

• heavy machinery

• structural connections

• industrial plant equipment

They offer significantly higher load capacity than Grade 8.8 bolts.

Grade 12.9 Bolts

Grade 12.9 bolts are ultra high-strength fasteners used in precision engineering.

Typical applications include:

• heavy mechanical systems

• tooling and dies

• high-performance machinery

• industrial drives

These bolts are extremely strong but may also be more brittle, requiring careful installation.

Bolt Head Markings Explained

Metric bolts typically have their strength class stamped on the head.

Common markings include:

These markings help installers confirm the correct fastener grade during assembly or maintenance.

If no grade marking is visible, the bolt may be low-strength or non-standard.

Bolt Grades vs Bolt Materials

Bolt strength is influenced by both material composition and heat treatment.

Common bolt materials include:

Carbon Steel Bolts

Used for most industrial fasteners.

Alloy Steel Bolts

Used for high tensile grades such as 10.9 and 12.9.

Stainless Steel Bolts

Offer superior corrosion resistance but typically lower strength.

Material selection depends on:

• load requirements

• corrosion exposure

• engineering standards

Bolt Grades and Torque Requirements

Higher strength bolts require greater tightening torque to achieve the correct clamping force.

Incorrect torque can cause:

• joint loosening

• bolt fatigue

• structural failure

Refer to our Metric Bolt Torque Chart for recommended tightening values.

Choosing the Correct Bolt Grade

When selecting a bolt grade, engineers should consider:

Load Capacity

Higher loads require stronger bolts.

Safety Factors

Structural connections require appropriate safety margins.

Environmental Conditions

Outdoor applications may require corrosion resistant coatings.

Industry Standards

Engineering specifications often define minimum bolt grades.

Frequently Asked Questions

What is the strongest metric bolt grade?

Grade 12.9 is one of the strongest commonly used metric bolt grades.

What does 8.8 mean on a bolt?

The marking 8.8 indicates:

• tensile strength = 800 MPa

• yield strength = 640 MPa

Is a 10.9 bolt stronger than 8.8?

Yes. Grade 10.9 bolts are significantly stronger and designed for higher load applications.

Are higher grade bolts always better?

Not necessarily. Higher strength bolts may be more brittle and require precise torque settings.

Shop High Tensile Bolts

Konnect Fastening Systems supplies a comprehensive range of industrial fasteners and high tensile bolts, including:

• hex bolts

• socket head cap screws

• countersunk socket screws

• structural bolts

• nuts and washers

All products meet relevant engineering standards and are suitable for construction, fabrication and industrial applications.