What Do Screw Grades 4.8, 8.8, 10.9, and 12.9 Mean?

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Last updated: 2 Jul 2026

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What Do Screw Grades 4.8, 8.8, 10.9, and 12.9 Mean?

When buying screws or bolts, you may notice markings on the head such as 4.8, 8.8, 10.9, or 12.9. Many people see these numbers but are not sure what they actually mean.

These numbers are known as the property class or strength grade of metric screws and bolts. They indicate the mechanical strength of the fastener, especially tensile strength and yield strength.

Choosing the right screw grade is important because each grade is designed for different load requirements. If the grade is too low for the application, the screw may stretch, deform, or break. On the other hand, choosing a grade that is much higher than necessary may increase cost without adding real value to the application.

What Is a Screw Grade?
A screw grade is a classification that indicates the mechanical strength of a screw or bolt. It is commonly marked on the head of metric fasteners, such as:

4.8
8.8
10.9
12.9

These markings are not just product codes. They relate to mechanical properties such as:

Tensile strength
Yield strength
Resistance to permanent deformation
Suitability for light-duty, general-duty, or high-strength applications

How to Read 4.8, 8.8, 10.9, and 12.9
A metric screw grade has two parts: the number before the decimal point and the number after the decimal point.

For example: 8.8

The first number, 8, indicates a nominal tensile strength of approximately 800 MPa
The second number, 8, indicates that the yield strength is approximately 80% of the tensile strength
Therefore, a grade 8.8 screw has a nominal yield strength of about 640 MPa

In simple terms:
The higher the first number, the higher the tensile strength.
The higher the second number, the higher the resistance to permanent deformation.

Comparison Table of Screw Grades 4.8, 8.8, 10.9, and 12.9

Screw Grade	Approx. Tensile Strength	Approx. Yield Strength	Common Applications
4.8	400 MPa	320 MPa	General use, light-duty applications
8.8	800 MPa	640 MPa	Machinery, structures, general industrial use
10.9	1,000 MPa	900 MPa	High-load applications, automotive, heavy machinery
12.9	1,200 MPa	1,080 MPa	Very high-strength applications, molds, precision machinery

Note: These values are general property-class references. Actual selection should also consider screw size, thread type, material, coating, standard, and working environment.

1. What Is Grade 4.8?
Grade 4.8 is a general-purpose steel screw grade. It is suitable for applications that do not require high strength, such as light assembly work, general installation, furniture, and non-critical fastening applications.

The main advantage of grade 4.8 screws is that they are easy to use, economical, and suitable for many general-purpose tasks.

Suitable Applications
General installation work
Light assembly
Furniture
Light-duty fastening
Cost-sensitive applications

Precautions
Grade 4.8 screws are not recommended for heavy machinery, high-load applications, or areas with strong vibration or high tensile stress.

2. What Is Grade 8.8?
Grade 8.8 is one of the most commonly used grades in industrial applications. It offers higher strength than general-purpose screws and is suitable for machinery, steel structures, industrial assembly, factory maintenance, and applications that require stronger fastening.

Grade 8.8 bolts are often referred to as high tensile bolts because they provide better tensile and fastening strength than lower-grade screws.

Suitable Applications
General machinery
Some steel structure applications
Industrial assembly
Factory maintenance
Applications requiring stronger fastening than general screws

Precautions
Although grade 8.8 is stronger than general-purpose screws, size, length, thread type, and surface coating must still be selected correctly. For outdoor or humid environments, corrosion protection should also be considered.

3. What Is Grade 10.9?
Grade 10.9 is a high-strength screw grade designed for applications that require greater mechanical strength than grade 8.8. It is often used in automotive applications, heavy machinery, components under high tensile load, and fastening points exposed to shock or vibration.

This grade should be installed correctly, with suitable tightening torque and compatible nuts or washers.

Suitable Applications
Automotive applications
Heavy machinery
High tensile load applications
Vibration-prone areas
Industrial applications requiring high strength

Precautions
Grade 10.9 should not be selected only because it is “stronger.” Some applications may not require this level of strength. It is important to check whether the workpiece material, nut, washer, and tightening torque are suitable for the fastener grade.

4. What Is Grade 12.9?
Grade 12.9 is a very high-strength screw grade. It is commonly used in applications requiring exceptional strength, such as molds, precision machinery, specialized equipment, and high-load components.

Grade 12.9 is often found in socket head cap screws and precision fastening applications where space is limited but high clamping strength is needed.

Suitable Applications
Mold and die applications
Specialized machinery
Very high-strength fastening
Limited-space installation
Precision mechanical assembly

Precautions
Grade 12.9 offers high mechanical strength, but proper installation is important. Tightening torque, compatible nuts or components, and working environment must be considered. A high grade does not automatically mean better corrosion resistance.

A Higher Grade Does Not Always Mean Better for Every Application
Many people assume that a higher screw grade is always better. In reality, the right grade depends on the application.

For example, a general installation job may not require grade 12.9 because it may be unnecessarily expensive. On the other hand, using a low-grade screw in a high-load machine application may lead to failure.

When choosing a screw grade, consider:

Load or force requirements
Screw size and length
Thread type
Workpiece material
Working environment
Surface coating or corrosion protection
Proper tightening torque

Are Higher-Grade Screws More Rust-Resistant?
Not necessarily.

Grades such as 8.8, 10.9, and 12.9 mainly indicate mechanical strength. They do not automatically mean the screw has better corrosion resistance.

Rust resistance depends on other factors, such as:

Fastener material
Surface coating, such as zinc plating, black oxide, or hot-dip galvanizing
Environmental exposure
Storage conditions

For outdoor, humid, or coastal environments, both strength grade and corrosion protection should be considered.

What to Check Before Buying Screws
Before ordering screws, it is important to check:

Required screw grade: 4.8, 8.8, 10.9, or 12.9
Diameter and length
Metric or inch thread system
Head type, such as hex head, socket head, or button head
Indoor, outdoor, or humid working environment
Required surface coating
Compatible nuts and washers

Checking these details helps reduce ordering mistakes, save time on site, and ensure the fastener is suitable for the application.

Summary
Screw grades 4.8, 8.8, 10.9, and 12.9 indicate the mechanical strength of metric screws and bolts. Grade 4.8 is suitable for general use, grade 8.8 is commonly used in industrial and machinery applications, grade 10.9 is suitable for high-load applications, and grade 12.9 is designed for very high-strength and precision applications.

However, screw selection should not be based on grade alone. Size, thread type, material, surface coating, environment, and application requirements should also be considered.

SRR Fastener supplies a wide range of screws, nuts, washers, and fasteners

for general work, factory use, machinery, and industrial applications. If you are unsure which screw grade is suitable for your work, you can contact us for product recommendations that match your application.

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