Drawings & TolerancesThreads and standard parts

Bolt property classes — 8.8, 10.9, 12.9 and the rest

What the two numbers on a bolt head mean, the minimum strength, proof stress and hardness of every ISO 898-1 class, how bolts are marked and matched with nuts, and how stainless classes such as A2-70 differ.

LiveUpdated 11 October 20265 min read

Property class 8.8 and its nut

Class 8.8: nominal tensile strength and yield strength: 800 / 640MPa
Minimum tensile strength of 10.9 (nominal 1000): 1040MPa
Proof load of an M10 bolt of class 8.8: 33.6kN
Tensile stress area As of M10: 58.0mm²

The two numbers stamped on a steel bolt head are a compact strength specification. They come from ISO 898-1, which sets the mechanical properties of bolts, screws and studs made of carbon steel and alloy steel. The current edition is ISO 898-1:2013, the fifth, with a technical corrigendum issued the same year; ISO confirmed it in 2025 and a revision is under way.

Reading the two numbers

Take 8.8:

The first number × 100 is the nominal tensile strength in MPa: 8 → 800 MPa.
The second number is ten times the ratio of nominal yield strength to nominal tensile strength: .8 → 0.8.
So first × second × 10 gives the nominal yield strength: 8 × 8 × 10 = 640 MPa.

In the same way 10.9 means 1000 MPa and 900 MPa, and 12.9 means 1200 MPa and 1080 MPa.

8first number × 100800 MPanominal tensile strength Rm

8second number ÷ 100.8ratio of yield to tensile

=first × second × 10640 MPanominal yield strength

800 / 800 MPa

Rp0.2

640 / 640 MPa

05001000

nominalminimum (use for design)row d ≤ 16 mm

Matching nut class (ISO 898-2): 8

The designation taken apart for five classes, with the bolt head marking. The bars set the nominal values of the designation against the minimum values of the table below; for 8.8 the row for d ≤ 16 mm is used. The matching nut class follows ISO 898-2.

Minimum properties by class

minimum yield: ReL, Rpf or Rp0.2minimum tensile strength Rm,min

Minimum tensile strength and minimum yield (lower yield ReL, 0.2 % proof strength Rp0.2, or Rpf for the full-size fastener) by property class, ISO 898-1:2013.

Class	Rm nominal (MPa)	Rm min (MPa)	Yield min (MPa)	Proof stress Sp (MPa)	Elongation A min (%)	Hardness (HV)
4.6	400	400	ReL 240	225	22	120–220
4.8	400	420	Rpf 340	310	—	130–220
5.6	500	500	ReL 300	280	20	155–220
5.8	500	520	Rpf 420	380	—	160–220
6.8	600	600	Rpf 480	440	—	190–250
8.8, d ≤ 16 mm	800	800	Rp0.2 640	580	12	250–320
8.8, d > 16 mm	800	830	Rp0.2 660	600	12	255–335
9.8, d ≤ 16 mm only	900	900	Rp0.2 720	650	10	290–360
10.9	1000	1040	Rp0.2 940	830	9	320–380
12.9	1200	1220	Rp0.2 1100	970	8	385–435

Notes on the table:

For 4.8, 5.8 and 6.8 the standard marks the Rpf values as under investigation and gives them only to calculate the proof-stress ratio. Instead of A, measured on a machined test piece, it specifies the elongation of the full-size fastener Af: 0.24, 0.22 and 0.20.
8.8 has two rows: above 16 mm the standard sets slightly higher minimum values. 9.8 is defined only up to 16 mm.
The proof load of a bolt is Sp × A_s, where A_s is the stressed area of the thread. The bolt must carry it without permanent elongation. For an M10 coarse thread, A_s = 58.0 mm², so an 8.8 bolt has a proof load of 580 × 58.0 ≈ 33.6 kN and a minimum tensile load of 800 × 58.0 ≈ 46.4 kN.

Two routes to 12.9

ISO 898-1 allows class 12.9 to be made in two ways, with the same mechanical properties; its tables list the class twice, once with the designation underlined. One route is alloy steel, quenched and tempered at 425 °C or more. The other is carbon steel with additives such as boron, manganese, chromium or molybdenum, quenched and tempered at 380 °C or more.

The standard also advises caution when class 12.9 is considered: the manufacturer’s capability, the service conditions and the tightening method should be taken into account, and the environment can cause stress corrosion cracking, in both uncoated and coated fasteners.

Marking

Hexagon and hexalobular head bolts and screws with a thread diameter of 5 mm or more must carry the manufacturer’s identification mark and the property class, for all classes.
The dot may be left out: “88” and “109” are valid markings.
A leading zero, such as 08.8 or 010.9, marks a fastener with reduced loadability: it may not carry the full loads specified for its class.

Matching nuts

Nuts follow ISO 898-2, whose current edition is ISO 898-2:2022. Its property classes are 04, 05, 5, 6, 8, 10 and 12; class 9 has been deleted. A nut’s class number is the first number of the strongest bolt class it is designed to carry:

Bolt	Nut
5.8	5
6.8	6
8.8	8
10.9	10
12.9	12

A nut of a higher class may replace one of a lower class.

Stainless steel bolts

Stainless fasteners follow ISO 3506-1, current edition 2020, and are marked differently: steel group and grade, a dash, then the property class.

The letter gives the structure: A austenitic, C martensitic, F ferritic, D duplex. The digit after it is the alloy type within the group.
The two digits after the dash are one tenth of the minimum tensile strength.

Marking	Meaning	Rm min (MPa)	Rp0.2 min (MPa)
A2-70	Austenitic (1.4301 type, no Mo), cold worked	700	450
A4-80	Austenitic with 2–3 % Mo (1.4401 / 1.4404 type), “acid-resistant”	800	600
A2-50, A4-50	Austenitic, class 50	500	210

An A2-70 bolt is therefore weaker than an 8.8 steel bolt, and its yield is a smaller share of its tensile strength (450/700 ≈ 0.64, against 640/800 = 0.8). Do not swap one for the other without recalculating the joint.

Sources (14)

Every value and quotation on this page comes from the documents below.