Bolt grades and sizes: 8.8, 10.9, 12.9, A2 and A4, and metric hex bolt dimensions

What the property classes 4.8, 8.8, 10.9 and 12.9 and stainless A2-70 and A4-70 mean, minimum strength and tensile loads, metric thread pitch and hex head sizes, DIN 933 vs DIN 931, and which nut goes with which bolt.

Updated October 17, 2026

A metric bolt is described by its thread, length, head type and property class. The property class, stamped on the head, tells you how strong the bolt is. Choosing a class that is too low risks a failed joint; choosing one that is too high adds cost and, with some coatings, the risk of brittle failure. This guide explains the classes and gives the dimensions needed to order bolts and nuts.

What the numbers on a bolt head mean

For carbon and alloy steel bolts to ISO 898-1 the class has two numbers. The first × 100 is the nominal tensile strength in MPa; the second × 10 is the percentage of that strength at which the bolt yields. An 8.8 bolt has a nominal tensile strength of 800 MPa and yields at 80% of it, about 640 MPa.

Stainless steel bolts: A2-70 and A4-70

Stainless bolts to ISO 3506 are marked with the steel group and a strength class. A2 is 304-type steel, A4 is 316-type with molybdenum for better resistance to chlorides. The number after the dash is one tenth of the tensile strength: A2-70 and A4-70 have at least 700 MPa, A4-80 at least 800 MPa. A2-70 is about as strong as class 8.8 in tension but yields earlier, so it does not replace 8.8 in a structural design without a check. See 304 vs 316 stainless steel.

Metric bolt sizes: thread pitch, head and tensile load

• Across flats — the spanner size. DIN 933 bolts use 17, 19 and 22 mm for M10, M12 and M14 and 32 mm for M22; ISO 4017 uses 16, 18, 21 and 34 mm. Check which standard your tools and drawings expect.
• Fine threads — such as M10 × 1.25, M12 × 1.5 or M16 × 1.5 — are ordered with the pitch stated. Without a pitch, a metric thread is coarse.
• Minimum load — the tensile force the bolt must carry before breaking. Design loads are much lower and follow the code that applies, such as EN 1993-1-8 for steel structures.

DIN and ISO designations

A fully threaded bolt (DIN 933 / ISO 4017) suits short bolts and clamping parts of varying thickness. A partially threaded bolt (DIN 931 / ISO 4014) has a plain shank that carries shear better and is used where the bolt passes through the joint, as in steel connections.

Which nut goes with which bolt

• Class 8.8 bolt — nut class 8 or higher.
• Class 10.9 bolt — nut class 10. Use a class 12 nut only where the design asks for it.
• A2-70 or A4-70 bolt — stainless nut of the same steel group and strength class, to avoid galvanic corrosion.
• Hot-dip galvanized bolts — need nuts tapped oversize after galvanizing; ordinary nuts will not run on.
• A nut of a lower class than the bolt strips before the bolt breaks and reduces the strength of the joint.

Finishes

Plain (black, oiled) bolts are for indoor use or parts that will be painted. Zinc plated bolts suit dry indoor and lightly exposed use. Hot-dip galvanized bolts last much longer outdoors and are standard for galvanized steel structures. Electroplating 10.9 and 12.9 bolts carries a risk of hydrogen embrittlement, so these are usually supplied plain, with zinc flake coating or hot-dip galvanized under controlled conditions.

Fasteners we supply

• Hex bolts — DIN 933 class 8.8, DIN 933 class 10.9 and DIN 931 partially threaded class 8.8.
• Nuts — hex nuts class 8 and class 10, plus flat washers.
• Structural — DIN 6914 HV bolts class 10.9 and DIN 7990 construction bolts.
• Stainless — A2 hex bolts and A4 hex bolts; threaded rod class 8.8.

Frequently asked questions

Send your fastener list with standard, thread, length, class and finish in a quote request. EN 10204 3.1 certificates are available on request.