Bolt Grades 8.8, 10.9 and 12.9 Explained: What the Numbers Mean

Look at the head of an industrial bolt and you will usually find a small marking such as 8.8, 10.9 or 12.9. Those numbers are not decoration or a batch code. They tell you how strong the bolt is, and whether it can safely do the job you are asking of it. Using the wrong class is one of the most common and most avoidable causes of bolted joint failure.

This guide explains how to read the numbers, how the three common classes compare, and how to choose between them.

What a property class is

For metric carbon and alloy steel bolts, screws and studs, the strength classes are defined in the international standard ISO 898-1. These classes are called property classes, and they are written as two numbers separated by a dot.

• The first number, multiplied by 100, gives the nominal minimum tensile strength in megapascals (MPa). So "8" means about 800 MPa.
• The second number, divided by 10, gives the ratio of yield strength to tensile strength. So ".8" means the yield strength is about 80 percent of the tensile strength.
• Multiplying the two numbers by 10 gives the nominal yield strength. For 8.8, that is 8 x 8 x 10 = 640 MPa.

Tensile strength is the stress at which the bolt would finally break. Yield strength is the stress at which it starts to stretch permanently. In a bolted joint you want the bolt to stay in its elastic range, so yield strength matters just as much as the breaking figure.

The three classes at a glance

The nominal values for the three most common high-strength classes are:

• Class 8.8: tensile strength about 800 MPa, yield about 640 MPa.
• Class 10.9: tensile strength about 1000 MPa, yield about 900 MPa.
• Class 12.9: tensile strength about 1200 MPa, yield about 1080 MPa.

The standard sets slightly different minimum figures depending on bolt size, so always use the values in the standard or the manufacturer's data sheet for calculations. The numbers above are for understanding the relationship between the classes.

Class 8.8

Class 8.8 is the everyday structural and engineering bolt. It is usually made from medium carbon steel that is quenched and tempered, and it has a typical hardness of roughly 22 to 32 on the Rockwell C scale. It offers a good balance of strength, toughness and cost, which is why it is stocked in almost every size. You will find it in steel structures, machinery frames, vehicle components, brackets and general construction. If a drawing says "high-tensile bolt" without any further detail, it often means 8.8.

Class 10.9

Class 10.9 is made from alloy steel or medium carbon steel that is quenched and tempered to a higher strength. Hardness typically sits around 32 to 39 HRC. It is used where the joint carries heavier loads or needs a higher clamping force within the same space: engine and gearbox parts, heavy machinery, crane and lifting equipment, pressure-related flanges and high-load structural connections. A 10.9 bolt of the same size carries around 25 percent more tensile load than an 8.8.

Class 12.9

Class 12.9 is the highest class in common use. It is made from alloy steel, quenched and tempered, with a typical hardness of about 39 to 44 HRC. Socket head cap screws, often called Allen bolts, are commonly supplied in 12.9, and it is widely used in tooling, moulds, machine building, hydraulics and precision assemblies where space is limited and clamping force must be high. The very high hardness has trade-offs, which we cover below.

A worked example: what the numbers mean in kilonewtons

Consider a standard M12 coarse-thread bolt, which has a tensile stress area of about 84.3 square millimetres. The proof load is the load the bolt must carry without any permanent stretch. Using the proof stresses given in ISO 898-1:

• 8.8: 580 MPa x 84.3 mm² gives a proof load of about 49 kN.
• 10.9: 830 MPa x 84.3 mm² gives about 70 kN.
• 12.9: 970 MPa x 84.3 mm² gives about 82 kN.

These are not working loads. A bolt is normally tightened to a fraction of its proof load, and the design must also account for shear, fatigue, safety factors, and the strength of the parts being joined. Treat the figures as a way to compare classes, not as a design table.

Why higher is not always better

It is tempting to assume that the highest grade is always the safest. In practice, class 12.9 is not a free upgrade. Several things change as strength rises.

Less ductility and more sensitivity to notches

Harder steel is less tolerant of scratches, thread defects, misalignment and sharp corners. A 12.9 bolt that is bent or poorly seated can fail suddenly without much warning, whereas an 8.8 bolt will usually stretch first.

Hydrogen embrittlement

High-strength steel can absorb hydrogen during acid cleaning or electroplating. Over time, this can cause delayed cracking under tension. The risk increases with hardness, so electroplated 12.9 bolts need strict process control, such as baking, or a different finish such as a zinc flake coating. For more detail on coatings, read our guide on HDG, GI and zinc-plated fasteners.

Mismatch with the surrounding material

A very strong bolt screwed into a weaker female thread, such as cast aluminium, soft steel or plastic, will strip that thread long before the bolt is stressed. The joint is limited by the weakest part, not the strongest.

Cost and availability

Higher classes cost more, and some sizes or lengths are less readily available. If an 8.8 bolt of a slightly larger size meets the requirement, it may be the more practical and economical answer.

Match the nut to the bolt

A bolt is only as strong as the nut that goes on it. Nuts have their own property classes in ISO 898-2, shown as a single number: 8, 10 or 12. As a general rule:

• An 8.8 bolt pairs with a class 8 nut, or higher.
• A 10.9 bolt pairs with a class 10 nut, or higher.
• A 12.9 bolt pairs with a class 12 nut.

Using a class 5 or low-strength nut on a 10.9 bolt can cause the nut threads to strip before the bolt reaches its capacity. Always check the nut marking, and buy bolts and nuts as matched sets.

How to read the markings

Metric bolts of the common classes carry the property class on the head, for example "8.8" or "10.9", along with a manufacturer's mark that identifies the maker. Studs and set screws may carry markings on the end face or a symbol system. Some points to remember:

• A bolt with no markings is typically a low-strength commercial grade, such as 4.6, or an unknown product. Do not use it for structural or safety-critical work.
• Stainless steel bolts are marked differently, for example A2-70 or A4-80. Read more in our article on SS304 vs SS316.
• If markings look rough, uneven or oddly worn, be cautious. Fake and mismarked fasteners are a known problem in the global market.
• Ask the supplier for a material test certificate if the joint is critical. Mufaddal Fasteners can supply certificates to EN 10204 3.1 on request.

Metric classes versus inch grades

Some equipment from the United States uses SAE grades instead of ISO classes. Grade 2, Grade 5 and Grade 8 are the common SAE ones, marked with radial lines on the head. As an approximate guide, SAE Grade 5 is in the same general strength range as class 8.8, and SAE Grade 8 is in the same general range as class 10.9. They are different standards with different details, so do not treat them as interchangeable without checking. Our article on DIN, ISO and ASTM fasteners explains how standards relate to each other.

Shear, tension and preload

Bolts do three different jobs, and the class affects each one differently.

• Tension: the bolt is pulled along its axis. Strength is proportional to the tensile stress area.
• Shear: the load acts across the bolt. Shear strength is typically around 60 percent of tensile strength, and it matters whether the shear plane passes through the threaded or the unthreaded portion of the shank.
• Preload: in many joints the real job of the bolt is to clamp the parts together with enough force that friction carries the load. The preload is created by tightening, so the tightening torque and the friction of the threads and under the head are as important as the class.

For critical joints, use a calibrated torque wrench and follow the specified tightening procedure. Do not use impact tools on bolts that need controlled preload unless the procedure allows it.

Practical advice for choosing a class

• General fixings, brackets, frames and non-critical joints: class 8.8 is usually adequate.
• Heavy machinery, high-load structural joints, crane and hoist components: class 10.9, as specified by the designer.
• Compact assemblies, tooling, precision machinery, socket head cap screws: class 12.9, where the design requires it and the finish and application are suitable.
• Corrosive or marine environments: consider A4 stainless steel, keeping in mind its lower strength than 10.9 or 12.9 carbon steel.
• Never downgrade: do not replace a specified 10.9 bolt with an 8.8 because it is available, or because it looks the same.
• Do not reuse high-strength bolts that have been tightened to yield or that show thread damage, especially in critical joints.

Where to find these in our catalogue

Our range includes the hex bolt, cap Allen bolt, flange bolt, stud bolt and B7 threaded rod, together with matching hex nuts and washers. For a more detailed look at our range, see hex bolts in Dubai. If you are unsure of the right class for your job, send the drawing or specification through the quote form and the Mufaddal Fasteners team will help you choose.