Bolt Grade Markings and Strength Chart

Head Marking Grade and
Material
Nominal Size Range
(inches)
Mechanical Properties
Proof Load
(psi)
Min. Yield Strength
(psi)
Min. Tensile Strength
(psi)

US Bolts


No Markings

Grade 2

Low or medium carbon steel

1/4 thru 3/4 55,000 57,000 74,000
Over 3/4 thru 1-1/2 33,000 36,000 60,000

3 Radial Lines

Grade 5

Medium Carbon Steel, Quenched and Tempered

1/4 thru 1 85,000 92,000 120,000
Over 1 thru 1-1/2 74,000 81,000 105,000

6 Radial Lines

Grade 8

Medium Carbon Alloy Steel, Quenched and Tempered

1/4 thru 1-1/2 120,000 130,000 150,000
Stainless markings vary. Most stainless is non-magnetic

18-8 Stainless

Steel alloy with 17-19% Chromium and 8-13% Nickel

All Sizes thru 1   20,000 Min. 65,000 Typical 65,000 Min. 100,000 – 150,000 Typical
Head Marking Class and
Material
Nominal Size Range
(mm)
Mechanical Properties
Proof Load
(MPa)
Min. Yield Strength
(MPa)
Min. Tensile Strength
(MPa)

Metric bolts


8.8

Class 8.8

Medium Carbon Steel, Quenched and Tempered

All Sizes below 16mm 580

640

800

16mm - 72mm 600 660 830

10.9

Class 10.9

Alloy Steel, Quenched and Tempered

5mm - 100mm 830

940

1040


12.9

Class 12.9

Alloy Steel, Quenched and Tempered

1.6mm - 100mm 970

1100

1220

Stainless markings vary. Most stainless is non-magnetic.

Usually stamped A-2

A-2 Stainless

Steel alloy with 17- 19% Chromium and 8-13% Nickel

All Sizes thru 20mm  

210 Min.
450 Typical

500 Min.
700 Typical

Tensile Strength: The maximum load in tension (pulling apart) which a material can withstand before breaking or fracturing.

Yield Strength: The maximum load at which a material exhibits a specific permanent deformation

Proof Load: An axial tensile load which the product must withstand without evidence of any permanent set.

1MPa = 1N/mm2 = 145 pounds/inch2