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Metric Bolt Property Class Chart (4.6 to 12.9, ISO 898-1)

The two numbers on a metric bolt head, such as 8.8 or 10.9, encode its strength. This reference lists the common property classes with their minimum tensile, yield and proof stress so buyers and engineers can compare options and spot a wrong substitute.

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Property class	Min tensile Rm (MPa)	Yield Re or Rp0.2 (MPa)	Proof stress Sp (MPa)	Yield / tensile (%)	Typical material and treatment
4.6	400	240	225	60%	Low or medium carbon steel
4.8	420	340	310	81%	Low or medium carbon steel
5.6	500	300	280	60%	Low or medium carbon steel
5.8	500	420	380	84%	Low or medium carbon steel
6.8	600	480	440	80%	Low or medium carbon steel
8.8	800	640	580	80%	Medium carbon steel, quenched and tempered (values for d up to 16 mm)
9.8	900	720	650	80%	Medium carbon steel, quenched and tempered (d up to 16 mm)
10.9	1,040	940	830	90.4%	Alloy or medium carbon steel, quenched and tempered
12.9	1,220	1,100	970	90.2%	Alloy steel, quenched and tempered

Values are the minimum figures in ISO 898-1 for bolts, screws and studs. Class 8.8 above M16 has a higher minimum tensile (830 MPa) and yield (660 MPa) and a proof stress of 600 MPa. Class 4.8, 5.8 and 6.8 yield values are nominal. Always confirm against the current edition of the standard and the product standard for the part.

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How to read the class number

The first number multiplied by 100 gives the minimum tensile strength in MPa, so class 8.8 starts at about 800 MPa. The second number is the yield-to-tensile ratio times 10, so 0.8 for the second digit 8 means yield is about 80 percent of tensile. Class 10.9 therefore means about 1000 MPa tensile and 90 percent yield, although the table shows the exact minima. Use the yield ratio column to see how much margin a class has before permanent stretch.

Using the values

Proof stress is the load the bolt must carry without permanent set in the test, and it is the basis for preload recommendations. Multiply the proof stress by the tensile stress area of the thread to get the proof load, then pick a preload at a stated percentage of it. Pair this with a thread stress area calculator and a torque specification. Pick the property class of the nut to match the bolt, for example class 8 or 10 nuts with 8.8 and 10.9 bolts.

Proof load (N) = Sp (MPa) x stress area (mm2)
Preload is commonly set at 60 to 75 percent of proof load for reusable joints
Class 12.9 is more sensitive to hydrogen embrittlement after plating
Do not heat or weld a quenched and tempered bolt

Common mistakes

Replacing a 10.9 bolt with an 8.8 because it is in stock is the most common one and is a design change that needs approval. Another is applying the torque of one class to another class or to a lubricated thread, where the same torque gives a much higher preload. Mixing inch grades such as SAE grade 5 or 8 with metric classes in the same ERP item record also causes wrong substitutions, so store the standard and class as separate attributes.

Frequently Asked Questions

What does 8.8 mean on a bolt?

It is the property class under ISO 898-1. The first 8 means a minimum tensile strength of about 800 MPa, and the second 8 means yield is about 80 percent of tensile, around 640 MPa for sizes up to M16.

Is 12.9 always better than 10.9?

It is stronger, but not always better. Class 12.9 is less ductile and more sensitive to hydrogen embrittlement and stress corrosion, so many designers use 10.9 unless the extra strength is needed.

Can I substitute a higher class bolt?

Not automatically. A stronger bolt can overload the clamped parts or tapped threads, and torque values change. Treat it as an engineering change and check the joint, the nut class and the preload.

What is proof stress?

Proof stress is the stress at which a bolt shows no more than a small permanent elongation in the proof load test. It is lower than yield and is used to define the proof load and set safe preload levels.

If fastener substitutions and torque specs are tracked across ERP, drawings and quality records, we can help connect them so a wrong class is flagged before it ships.

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