Bolt Grades 4.6, 8.8, 10.9 and 12.9: What the Head Marking Means Under ISO 898-1

The bolt grade stamped on a bolt head, such as 8.8 or 10.9, is a strength code defined by ISO 898-1. The first number times 100 gives the nominal tensile strength in MPa, and the second number gives the ratio of yield to tensile strength. So an 8.8 bolt has at least 800 MPa tensile and 640 MPa yield strength. For structural joints, the grade must follow the drawings rather than simply picking the strongest bolt.

Key Facts

• Class 4.6: minimum tensile 400 MPa, minimum yield 240 MPa, nominal proof stress 225 MPa (2026-10-27, source: HEXVARO, ISO 898-1:2013 property class table (reviewed 27 Oct 2026, accessed 3 Nov 2026)).
• Class 8.8: minimum tensile 800 MPa for d ≤ 16 mm and 830 MPa for d > 16 mm; minimum Rp0.2 yield 640/660 MPa (2026-10-27, source: HEXVARO, ISO 898-1:2013 property class table (reviewed 27 Oct 2026, accessed 3 Nov 2026)).
• Class 10.9: minimum tensile 1,040 MPa, minimum yield 940 MPa; class 12.9: 1,220 MPa and 1,100 MPa (2026-10-27, source: HEXVARO, ISO 898-1:2013 property class table (reviewed 27 Oct 2026, accessed 3 Nov 2026)).
• Hexagon head bolts with d ≥ 5 mm must carry the property class symbol and the manufacturer mark; the dot may be omitted (2026-10-27, source: HEXVARO, ISO 898-1:2013 property class table (reviewed 27 Oct 2026, accessed 3 Nov 2026)).

How to read a bolt grade

ISO 898-1:2013 sets the mechanical properties of bolts, screws and studs made of carbon steel and alloy steel. The class code has two numbers separated by a dot:

• First number × 100 = nominal tensile strength (MPa). An 8 means about 800 MPa.
• Second number × 10 = yield as a percentage of tensile strength. An 8 means yield is about 80% of tensile, i.e. 640 MPa.

Another example: 4.6 means 400 MPa tensile and 60% yield, or 240 MPa. Class 10.9 means a nominal 1,000 MPa with a 90% yield ratio; in the standard's table the tested minimums are 1,040 MPa tensile and 940 MPa yield.

Quick table of ISO 898-1 classes

• 4.6: tensile 400 MPa, yield 240 MPa, hardness 120–220 HV.
• 5.6: tensile 500 MPa, yield 300 MPa, hardness 155–220 HV.
• 8.8: tensile 800/830 MPa, yield 640/660 MPa, hardness 250–320 / 255–335 HV (d ≤16 / >16 mm).
• 10.9: tensile 1,040 MPa, yield 940 MPa, hardness 320–380 HV.
• 12.9: tensile 1,220 MPa, yield 1,100 MPa, hardness 385–435 HV.

The 8.8 values are split by diameter because larger bolts are tested to slightly higher figures. Proof stress is the load a bolt must carry without permanent deformation; for 8.8 it is 580/600 MPa.

Where the marking goes

For hexagon and hexalobular head bolts of 5 mm diameter and above, the head must show the manufacturer's mark and the class symbol. It is usually on top of the head, or indented on the side. The dot may be left out, so "88" or "109" is also common. A bolt without a class marking cannot be assumed to meet any particular class.

Do not judge by colour

Black, yellow or silver only indicates the surface finish, not strength. An 8.8 bolt can be galvanized, and an unrated bolt can look shiny. What counts is the head marking and the manufacturer's test documents.

Choosing a bolt grade for renovation

• For light work such as shelving, fences or timber brackets, 4.6 or 5.6 is usually enough unless the designer asks for more.
• For structural steel joints, canopies, steel stairs or machine bases, follow the grade on the drawings; 8.8 is widely used.
• Classes 10.9 and 12.9 are for high loads and need controlled tightening. A higher grade is not automatically better, because harder bolts tend to be less ductile.
• Pair bolts with nuts and washers of a matching class; nut classes are covered by a separate standard, ISO 898-2.
• When buying in volume, ask for a test certificate (mill certificate) for each lot.

homeX, an integrated building materials and furniture platform in Indonesia with showroom, warehousing and delivery services, can help source bolts, nuts and steel accessories to project specifications.

Bolt grade: related articles and products

• Nail and screw prices 2026
• BMT vs TCT light steel
• Rebar weight per metre SNI 2052
• homeX building materials catalog

FAQ

What does 8.8 on a bolt head mean? It is the ISO 898-1 property class: minimum tensile strength 800 MPa (830 MPa above 16 mm diameter) and minimum yield 640 MPa.

Which is stronger, 10.9 or 12.9? Class 12.9 is stronger, with minimum tensile 1,220 MPa versus 1,040 MPa for 10.9. Selection should still follow the designer and product standard.

Can unmarked bolts be used for structures? Better not. ISO 898-1 requires the class and manufacturer marks on hexagon bolts of 5 mm and above, so an unmarked bolt cannot be confirmed to any class.

For building and renovation material needs, reach the homeX team on WhatsApp +62 821 1818 998, by email at homex9998@gmail.com or through our contact page.

Sources: HEXVARO, ISO 898-1:2013 property class table (reviewed 27 Oct 2026, accessed 3 Nov 2026); ISO 898-1:2013, official sample via iTeh Standards (accessed 3 Nov 2026)