The numbers stamped on a bolt head, such as 4.6, 8.8 or 10.9, tell you how strong the bolt is. Reading them correctly helps you buy the right bolt for the job and avoid paying for strength you do not need, or worse, using a weak bolt where a strong one is required.

What the two numbers mean

Steel bolts are graded by property class under ISO 898-1 (adopted in India as IS 1367 Part 3). The class has two numbers separated by a dot:

• The first number × 100 gives the nominal tensile strength in N/mm² (MPa). This is the stress at which the bolt breaks.
• The second number ÷ 10 gives the ratio of yield strength to tensile strength. Yield is the point at which the bolt starts to stretch permanently.

Example, class 8.8: tensile strength 8 × 100 = 800 MPa. Yield = 800 × 0.8 = 640 MPa. In practice you can multiply the two numbers and then by 10 to get yield: 8 × 8 × 10 = 640.

Common grades at a glance

Nominal values are shown. Actual minimum values in the standard vary slightly with size, for example class 8.8 above M16.

How to spot the grade

• Hex bolts of class 8.8 and above must be marked with the class on the head, usually with the maker's mark.
• Unmarked bolts are usually class 4.6 or lower. Do not assume an unmarked bolt is high tensile.
• Socket head cap screws are commonly 12.9 and are marked on the head or the side.

Match the nut and washer to the bolt

A strong bolt is only as good as its nut. Under ISO 898-2 (IS 1367 Part 6) the nut class should be equal to or higher than the first number of the bolt class:

• Class 4.6 bolt: class 4 or 5 nut.
• Class 8.8 bolt: class 8 nut.
• Class 10.9 bolt: class 10 nut.
• Class 12.9 bolt: class 12 nut.

Use hardened washers under high-tensile nuts and bolt heads so that the bearing surface does not dig in and lose preload.

Stainless steel grades are different

Stainless bolts use a letter-and-number code under ISO 3506. A2 is equivalent to SS304 and A4 to SS316. The number after the dash is one tenth of the tensile strength, so A2-70 means A2 stainless with 700 MPa tensile strength. A2-70 and A4-70 bolts have a minimum yield strength of about 450 MPa, well below class 8.8, so do not substitute them for high-tensile steel bolts in a structural design without checking.

Structural (HSFG) bolts

High strength friction grip bolts to IS 3757, marked 8.8S or 10.9S, are made for pre-loaded structural steel connections. They come as a matched set of bolt, heavy nut and hardened washers and must be tightened by a controlled method. Use them only where the steel design calls for them.

Practical buying tips

• Do not mix grades in one joint. The joint behaves like its weakest bolt.
• Higher is not always better. Very high grades (10.9, 12.9) are less ductile, and when they are electroplated there is a risk of hydrogen embrittlement. Specify plating carefully or choose hot-dip galvanised 8.8 for outdoor structures.
• Give full details when ordering: diameter, length, thread pitch if not standard, grade, finish and whether you need a full or partial thread.

This guide is general information for buyers. Always follow the anchor or fixing manufacturer's data sheet and your project engineer's design for load-bearing and safety-critical work.