Fastener Strength Grades Explained: Choosing the Right Property Class
When sourcing bolts, screws or studs, the property class (strength grade) is one of the most important specifications. It defines the mechanical properties of the fastener — tensile strength, yield strength and hardness — and determines whether the part is safe for your application. This guide explains how to read strength grades and choose the right one for your project.
What is a property class?
Property classes are standardized numbers marked on the head of bolts and screws. In the metric system (ISO 898-1), common classes include 4.8, 5.8, 8.8, 10.9 and 12.9. The number before the decimal point, multiplied by 100, gives the nominal tensile strength in MPa; the number after the point, multiplied by 10, gives the ratio of yield strength to tensile strength.
Common strength grades at a glance
| Property Class | Material Type | Tensile Strength (MPa) | Typical Use |
|---|---|---|---|
| 4.8 | Low-carbon steel | 400 | General light-duty fastening |
| 8.8 | Carbon steel, quenched & tempered | 800 | General structural and machinery |
| 10.9 | Alloy steel, quenched & tempered | 1000 | High-strength applications |
| 12.9 | Alloy steel, quenched & tempered | 1200 | Extreme high-strength applications |
How to choose the right grade
Consider the working load, joint type and environment. For general machinery and furniture, 4.8 or 8.8 carbon steel is usually sufficient. For structural steel connections, vehicle parts and high-vibration applications, 10.9 or 12.9 alloy steel is required. Always match the nut property class to the bolt — for example, class 8 nuts pair with 8.8 bolts, and class 10 nuts pair with 10.9 bolts.
Stainless steel grades
Stainless steel fasteners use a different marking system: A2 (304) and A4 (316) are the most common. A2 suits general corrosion resistance; A4 offers superior resistance to chlorides and marine environments. Stainless fasteners are non-magnetic or slightly magnetic and are not heat-treated to the same strength levels as alloy steel.
Sourcing with confidence
Through FASTENER HALL you can source certified fasteners in all common strength grades, with material test reports, thread gauging and AQL inspection available on every order. Submit your RFQ with the required property class and our sourcing team will match you with the right certified factory.
