The numbers on a bolt head tell you how strong the bolt is. This guide explains metric property classes, shows the strength of each grade in one chart, and helps you choose the right bolt and nut for the job.

How to Read a Bolt Grade

Metric steel bolts are marked with a property class such as 4.6, 8.8, 10.9 or 12.9. The classes are defined in ISO 898-1, which India has adopted as IS 1367 (Part 3).

First number × 100 = nominal tensile strength in MPa (N/mm²). For 8.8: 8 × 100 = 800 MPa.
First number × second number × 10 = nominal yield strength in MPa. For 8.8: 8 × 8 × 10 = 640 MPa.

Tensile strength is the load at which the bolt breaks. Yield strength is the load at which it starts to stretch permanently. Bolted joints are designed to work below the yield strength.

Bolt Grade Chart

Property class	Tensile strength	Yield strength	Material	Typical use
4.6	400 MPa	240 MPa	Low or medium carbon steel	Light structures and general fabrication
5.6	500 MPa	300 MPa	Medium carbon steel	Foundation bolts and general engineering
8.8	800 MPa	640 MPa	Medium carbon steel, quenched and tempered	Machinery, structural steel and automobiles
10.9	1000 MPa	900 MPa	Alloy steel, quenched and tempered	Heavy machinery and high-load joints
12.9	1200 MPa	1080 MPa	Alloy steel, quenched and tempered	Dies, moulds and high-strength socket screws

These are nominal values. ISO 898-1 gives the exact minimum for each size; for example, an 8.8 bolt above M16 has a minimum tensile strength of 830 MPa.

Matching Nut Classes

A nut must be at least as strong as the bolt, or the thread can strip. ISO 898-2 sets the matching nut classes:

Bolt class	Matching nut class
4.6 and 5.6	5
8.8	8
10.9	10
12.9	12

A nut of a higher class can always be used with a bolt of a lower class, but not the other way round.

Stainless Steel Bolt Grades

Stainless steel bolts use a different marking, from ISO 3506-1. The letter and first number show the steel type, and the last number shows the strength:

Marking	Steel	Tensile strength	Yield strength
A2-70	SS 304	700 MPa	450 MPa
A4-70	SS 316	700 MPa	450 MPa
A2-80 / A4-80	SS 304 / SS 316, stronger	800 MPa	600 MPa

SS 202 is not covered by ISO 3506. It is an economical grade with less nickel, used for indoor and dry work. Our SS 304 vs SS 316 vs SS 202 guide compares the stainless grades in detail.

How to Choose the Right Grade

4.6 or 5.6: light structures, sheds, railings and foundation bolts where high strength is not needed.
8.8: the standard high tensile grade for machinery, structural steel and solar structures.
10.9: heavy machinery and joints with high load or vibration.
12.9: Allen cap screws in dies, moulds and machine tools. These are usually supplied black or plain, because electroplating very high-strength steel can cause hydrogen embrittlement.
Stainless steel: wherever rust is a concern. SS 304 for general use, SS 316 near the sea or chemicals.

Always follow the grade given in your drawing or design code.

Head Markings

Steel bolts are normally marked with the property class on the head, for example 8.8 or 10.9, along with the maker’s mark. Stainless steel bolts are marked A2-70, A4-70 and so on. If a bolt has no marking, ask the supplier for a test certificate.

Graded Bolts and Nuts from JMIKS Fasteners

JMIKS Fasteners supplies hex bolts, Allen cap screws, threaded rods and hex nuts in grades 4.6 to 12.9 and in SS 304, SS 316 and SS 202, from Ahmedabad. See the full bolts and machine screws range or send your requirement for a quotation.

Bolt Grade Chart: 4.6, 8.8, 10.9 and 12.9 Explained